get a sear, preferably in a mp5(hk94) host that runs, IF NOT get a sear then a german gun and have TSC or Jeff time/install.
enjoy the gun with the least amount of problems. get the mp5 before the k
DONT waste time with clones-just DONT
afa as the sear, once its installed in a pack it can be dropped into other guns(housings) easily. One thing you can't do is a sef pack into a ambi housing. just get a sef pack to start then get other guns, then worry about ambi packs later.
sp5s can be modified (how is still up in the air), if you get a sear now, by the time you get it they will have all the atf BS worked out, but i would still get a "shooter" 94 to start.
from there you can get other guns.
AFA as the beltfed, the cheapest would be to find a used TSC built german or fmp 21. They are not as nice as the 21/23es but they are cheaper and usually more reliable.
as a WARNING clone beltfeds can be a huge mess. tolerances/heat treating/ and how they are built can make or break your gun.
If you buy one, MAKE SURE YOU TEST IT BEFORE YOU BUY IT.
Cloners have produced many **** guns and it sucks having spent 15k+ on a junk gun.
My advice, buy something TSC has built. realize there are basically zero parts kits left and it will be a premium.
If you have to get a clone, I recommend researching them extensively. And don't do it on just this forum as its very biased.