What are the different options and relative advantages and disadvantages?  I'm looking at 5.56 and 7.62, with a preference for 5.56.

dias w/ shrike upper= fun but not a real belt fed
HK series= real beltfeds but more expensive

5.56 in belt form is cheaper and funner on the range

past that do a search, multiple postings about this

Sorry, should have specified I'm looking for HK sear options.

HK 21(E/K) (7.62x51mm)
HK 22(E/K)(7.62x39mm)
HK 23(E/K) (5.56mm)
HK 51B (k) (7.62x51mm)
HK G3B (k) (7.62x51mm)

K = short barrel
E = Upgraded/more modern components
B = Beltfed

You have a few options. Arguably the best is to find a 21E or 23E parts kit and an HK91 and have TSC build you an all German E gun. This is also the most expensive way. Given the current price on parts kits you're looking at $25K or more. 23E parts kits in particular are pretty rare, especially the barrels.

Beltfed Precision and Michaels Machines make 21E/23E clones for about half the cost of doing a parts kit build with TSC. There are a few people on this forum who own one or the other. Early BPP guns seemed to have heat treat issues, but this appears to have been resolved. Early MM guns that were made by Gerard before he died also seem to be pretty good.

I personally own a fleming sear and mm23e, and strongly advise against going this route. It's now over 2 years since I bought my mm23e and it still doesn't work. My original receiver also had some build quality issues, and other forum members have PM'd me with similar complaints. Additionally, the MM guns are not true E spec, since they cannot take E barrels. You will only find good reviews of MM on the hkpro forums because he's protected over there. Bad reviews or anything that speaks ill of MM are instantly deleted by the moderators.

there is a thread in the HK forum

Not really.  You can ask questions but the MM fanboys will provide glowing reviews and videos.
Slippers is correct in his assertion that the moderators on HKPro censor any negative comments about MM.

I believe he is referencing a belt fed thread in the HK forum here.  (where negative MM comments are not deleted)

https://www.ar15.com/forums/Armory/Take-me-to-belt-fed-school-Make-me-smart-on-21-23-s/9-483469/

Lots of good discussion here with pro/cons to each avenue.

From a cliff notes perspective for the OP.

My personal take is that unless you own at least one or two HK sears, a sear host is definitely the way to go as the sear can be used on a myriad of other HK hosts and don't lock you into a single "somewhat exotic" beltfed platform.

If you already have a sear or two than maybe you want to go the "purist" route and get a registered receiver that is basically an almost mechanical clone of an Oberndorf built gun.    Prior to a decade ago there wasn't a legal semi auto "E" swing down semi variant available, so almost all "E" conversions had to be done on registered receivers.   I am not 100% sure who got the semi "E" receiver approval but I believe Mike Otte submitted a sample and got an approval letter.

In terms of German or US Clone build, my impression based on the comments I have seen over the years is that both of the two primary US Clone builders (Belt Fed Precision & Michaels Machines) have put out some problematic guns and many that run well.   Nobody really knows what the percentage of good vs. bad US Clone Hosts are delivered to customers by each respective builder other than the two outfits that make them.

For the record I own a MM 23EK that runs flawlessly as long as its fed mil-spec M855/SS109.  That said, MM seems to have treated some of his customers who received problematic guns pretty poorly based on posts I have seen over the years on Subguns, Sturm, AR15, and even HKPro to a limited extent.   This lack of CS from MM definitely gives me pause about getting a second full size gun from them.

On the contrary I have never seen a single bad word posted about Mike Woodard at TSC machine.  A full German build by TSC is basically the gold standard by which all others are judged (except maybe Dyer).  The downside is the cost for an all German semi host "E" build is basically 2 to 3X what a US clone would cost.  You will have to weigh the risk of whether a US built clone is worth saving probably $15 to $20K.

I meant on here in the HK forum where MM guns are called out and users debunk the BS on HKpro.

Im well aware of them cutting/choping post, even mine where I posted pics of broke ass MM out of spec guns

ETA post above nailed it

Yeah I think the safest conclusion is that it’s hit or miss if you don’t just go all German through TSC or something.

Both BFP and MM have had unhappy customers.  My MM23ek runs like a raped ape, but it does have a few cosmetic issues (the welds aren’t perfect, front sight *slightly* canted to the right).  Mike can be a bit of a nuisance to deal with, because he doesn’t even come close to making delivery estimates, almost never answers emails, etc.  The product is probably expensive for what it is too, but then again, those guns that run run very well.

I’m happy with mine

This is the picture that got me started down the road to my own 21e.

DOntforget the lakeside machine lm7 and razorback. AMazing fun once you get them dialed in.
Unfortunately ever since I sold my registered receiver it sits there dusty since they're not compatible with a RLL.