I have been bitten by the 51 bug.  The problem is I need to sell off a few guns.  Should I get rid of  a few of the AK's or my UMP clone? And what are the ump's worth now it's a grease gun conversion.    

Keep the USC and do the UMP conversion with the actual UMP parts. These rifles are only going to increase in value and on top of it, they are true H&K's and not clones. Vector will keep producing the 51 so you'll still have time to buy one down the road if you really want it. H&K on the other hand isn't bringing anymore USC's into the country and I highly doubt you'll be able to readly find one in a year or so.

Ditto what Jim said.  I had my USC converted by TE and I will never sell it.

And Jim, every time I see your MAR it reminds me I have to send off my parts to Firing Line to get it built up.