While an HK sear would be a lot more money, there is so much more than shooting a MP5. I have one or more clones of MP5K, MP5K PDW, MP5K built with a full size receiver, MP5 SD, MP5K-40 PDW, MP5-40, MP5-40 SD, MP5K-10 PDW, MP5K-45X built with a full size receiver, full size MP5-45, the list goes on... There is also for 5.56X45 the 53K AR with a 5.1" barrel, that uses AR mags. Plus 53 AR, 33K AR, and 33, just for fun. I have far more in hosts than current market value for a sear. Why, because one HK conversion device can do all that and more.
If you like the AR but not in rifle caliber, my DIAS with a custom modified MGI magwell that uses modified Thompson mags (think converted Uzi mags) with a Ron Williams .45 direct impingement shoots as smooth as a 9mm roller locked gun with the knock down of .45 ACP. On the other end of the spectrum is my 20" water cooled upper with KNS spade grip lower with a Valkyrie Armaments belt feed conversion. Check YouTube under "500 rd burst". My wife uses the water cooled set up to burn 500 rds in about a minute. 200 rd SAW boxes and this set up on a MG42 tripod is light and easy to set up for a water cooled belt fed and you can stand or sit.
Other than that, I'd see about trading the AC556 for a M11/9 and a M10. There are big things brewing for the MAC family of RRs. The S.A.B.R.E. 5.56 upper fell on its face. It was way too heavy and AA/MAG Tactical couldn't bother to answer the phone. Mark my words, within the next five years, the M10 and/or M11/9 will be doing rifle calibers conversions and jump in value. YMMV.
Scott