First off, if the weapon doesn't have the FA carrier installed, you'll need one of those. I have no idea on the SW guns if they have a FA carrier or not - all my Vectors came with them from the shop.
If it's currently in rifle configuration (and you have other hosts), it may be cheaper for you to flip the gun now and have a MP5 pistol built to your specs from the get go, since you'll probably not want to deal with a 16" rifle barrel or fake can, and SBRing it is an additional expense (for the times you don't have the sear installed).
If it's a pistol config, disregard above.
Before buying anything in the spare parts department:
-Test with semi-pack it came with.
-Check bolt gap.
If both check out ok, then switch to registered sear/pack and go to town. If it works, buy the usual culprits of spare parts (mags, extractor/ejector/firing pin/etc).
If it doesn't, check shelf height. Shim if necessary to raise ejector (if FTEj). If anything else, and it works in semi, flip it and get a kit MP5 built to your specs or try to find a Vector (they're out there but up in price quickly). If you're determined to go down the replace parts and pray route, consult with Adam Webber (from whom you'll likely be buying parts and toys from anyway) for suggestions. Expect to be looking at bolt heads, etc.
My Vector PDW:
I should add that anyone who knows what a sear host is or asks about one is 99.99% likely to be legal. HK guns and modularity and the ability to use one sear/pack per gun is why they're so common.
Retards generally start babbling about "Want MP5 for $500, how do I make it go zoom", not sear hosts.