If you do a G36, you'll end up doing a UMP. If you do a UMP, you'll end up doing a G36. I believe that's how it goes.
Shop smart and know when a deal is a deal. Back then I only had the option to buy a grey SL8-1 so I had the extra step of dying parts, but I got that down pat. I did get my rifle for only $1K though. All together it was less then $1600- 1700 for the whole project (that included some trades). Then I was constantly checking GB for deals and scored a $350 SL8-1 receiver. So all my spares went into a long rifle build.
Now that some key parts like UMP mags are getting hard to come by, you might be spending a bit more than what it used to take to build one. I scored a local buy and got my USC for $1K and four mags for $100. Again, shop smart if you can. But know when a hard part to get is within your grasp. If you have money to burn, then you shouldn't have any problems.