Whats the cheapest trigger pack/lower combo to finish off a semi auto mp5? Not very familiar with HK stuff, so just wondering.

The cheapest option (by far) is to modify a G3 pack by swapping a 9mm ejector, shortening the rear tang, doing semi mods, and whatever you need to do for the front to fit on your gun.
G3 SEF plastic lower

The problem with doing the labor yourself, however, is that you'd need to be familiar with HK stuff. There are some really good tutorials out there, YouTube, etc.

Or buy one ready to go on: 9mm SEF semi-auto lower clipped & pinned

You could cruise the EE looking for parts. I list some from time to time as I help friends get their guns 922r compliant. I have a spare complete semi Turk SEF trigger pack off of a new MKE gun I could let go for really cheap if you don't need to add 922r parts... you'd just need to find a clipped and pinned SEF lower to put it in.

If you need 922r and prefer new, you might want to check out the compatibility lowers HKparts has on offer.

Will a HK33 lower work in an mp5?

Yes, you just need to swap for a 9mm ejector. You may need a 9mm hammer spring also, but usually the 5.56 spring works fine.

It will have a 223 ejector that would need to be swapped out for a 9mm one. It could also have a 33/93 'single x' hammer and spring, but that might work with a 9mm carrier ok. The trigger housings are effectively identical (c&p withstanding).