I have an DD MK18 upper with the larger gas port and currently using the buffer that came with the PSA pistol lower.

I think I should use a H2 or maybe even an H3.  Also, would a different buffer spring also help and if so which one?

Thanks in advance.

If it runs well don't change a thing.

Any idea what buffer you have currently? I'd assume it's a standard carbine buffer. If that's the case, your internals are taking an unnecessary beating - even if it does run.

You should be able to go up to an H3 using a standard carbine spring without any issue. It'll still run on pretty much all ammo (maybe Tula would be an issue, but probably not), and smooth out the recoil impulse a bit. If you're shooting Tula type, very lower powered .223, you might want to try an H2. Unless you have an H2 or H3 on hand, I'd suggest grabbing 3 tungsten weights and a steel weight from KAK; it'll give you all you need to make an H to H3 using your current buffer body, and for about the price of a single buffer. That way you aren't locked into a single buffer and can always back down to an H2 if needed.

Once you know that an H3 will run with your setup and ammo, you always have the option of trying out some of the upgraded springs from Sprinco (Blue or Red). I've heard of people going all the way up to a Sprinco Red and H3 with the DD .082 Mk18 uppers, but YMMV.

h buffer is the default for a mk18 upper.

I assume you're referring to complete commercial DD guns that run an Mk18 upper? If DD did supply .082 uppers with an H, that's simply so it will run crap like Tula, not because it's remotely optimal. An H wouldn't be a bad choice for the current .070 spec uppers, but not that great for the old .082 GP. Military issue CQBR uppers run an H2, and those have a milspec .070 gas port (but also use full pressure 5.56 exclusively). Actually, a .070/H2 combo will also run most brass cased .223 no problem - just not the weakest stuff like Tula.

Bottom line: A .082 gas ported DD Mk18 upper can easily function using an H3 with most ammo (including Wolf WPA .223). If you shoot Tula, then you might need to back down to an H2.

Use the heaviest buffer that it will run with reliably and still have the bolt lock back on the last round fired.

Did you contact DD? Last I heard they were swapping the older barrels for the revised .07". No charge. I'd say take advantage of it and they had some decent turn around times.

This.
People have been obsessed with buffers the last few years. Heavy buffer seems to be the answer to every problem and if there is no problem put a heavier buffer anyway.
The movement of the buffer weights inside the buffer is actually much more important and has a bigger impact on the gun cycling than the overall weight of the buffer. Some heavy buffers will have greater overall weight by adding weights to the buffer in place of spacers or slide space and the weights are unable to slide inside the buffer as much or at all, this is a bad compromise. Heavier buffers that use Tungsten (as they should) should still have room for the weights to slide.
If someone wants to change the weights to Tungsten or Lead or change the entire buffer it can and does make a slight difference but not as much as many people think it does, and often a negative effect.  Standard buffers exist and are the norm because they are the weight that generally works the best overall. In many cases people will stick in a heavier buffer because they read it is an upgrade and their AR is now heavier, less reliable and cycles harder than it originally did. Experimenting with buffers and weights is fine but do not assume heavier is better. There are situations where a heavier buffer can quickly and easily remedy an AR that has problems and this is the reason they exist.
Throwing heavy buffers at every AR, even the ones that work just fine is not needed. If your AR works fine I would leave it alone. If you do change the buffer make sure you pay attention to how it feels as it cycles and how it reacts to all types of ammo. Reliability and smoothness of the gun are more important than if your buffer is ''heavy'' and you may very well  find the standard carbine buffer is the upgrade.

Of course any advice on buffer weights assumes you're purchasing or building a properly functioning buffer. That means either purchasing from a reputable company or building it yourself. I don't know where you buy your buffers, but I've never bought one that didn't use proper tungsten weights or that lacked the necessary weight movement. A heavier buffer that is properly made is never going to result in a weapon that cycles harder. It might make it function as if it's under gassed, but it's not going to have the opposite effect.  It also can make more than a slight difference. The correct buffer can mean the difference between a reliable gun with a smooth recoil impulse versus a heavily over gassed, unreliable one that is trying to rip the cases far too early and has unnecessarily sharp recoil. That, or an under gassed gun that is short stroking and barely cycling. Selecting the correct buffer shouldn't be a mysterious process.

The OP is asking about a specific upper with a known gas port size that has certain requirements to function optimally. Sure, it might cycle with an H buffer or even a carbine buffer (depending on ammo), but that's not going to produce the best results. While there is definitely some overlap - most guns that will function with an H3 will also function with an H2, for example - "functional" doesn't necessarily mean "optimal". Personally, I tune my guns to the maximum weight that will run because it produces a weapon that is both reliable and as pleasant to shoot as possible. And having a buffer on either extreme end, whether too light or too heavy, is equally undesirable. It does make a difference. If you're running a carbine buffer in an over gassed upper like the old .082 spec DD Mk18 - assuming it functions with any degree of reliability at all - you're forcing the gun to unlock at a much higher chamber pressure than is necessary. This effect will even more exaggerated if you're shooting hot 5.56 and/or suppressed. That means faster wear on components, a harsher "felt recoil" impulse and generally makes no sense. You'd be battering yourself and your components for no good reason.

Obviously it isn't advisable to randomly start throwing heavier buffers into your AR without understanding why you're doing it; that isn't likely to produce good results, and is probably what encouraged manufacturers like Daniel Defense to start using oversized ports like .082 in the first place. A properly tuned 10.3" with a .070 GP will run everything from hot 5.56 to steel case; the whole "a larger port is needed to run commercial ammo reliably" philosophy probably got started because of uninformed users thinking every short barrel needed an H2 or H3. Be that as it may, this particular upper actually does pair best with an H2 or H3.