SGF is correct about the receiver wear. I put somewhere in the range of 20,000 rounds through my Uzi and the receiver is much the same as the M11/9... Folded sheet metal. I'm probably close to ten thousand rounds through my current M11/9 with no visible signs of lower receiver wear.
SGF is also correct about the buffers. They're cheap insurance against heavy bolt contact with the rear of the registered receiver which will eventually break the welds. Go with neoprene rather than black rubber buffers which don't last long in my experience. In any case, I'd replace the buffer on any gun I bought before firing it.
The entire MAC family of subguns are extremely simple with very few parts to break or go bad. If you do have any issues, Sam Snyder of Practical Solutions is considered the MAC Guru when it comes to MAC trouble shooting and repair. I had Sam refinish one of my M11/9s and the work and turnaround time was great.
http://www.practicalpage.com/
The parts to watch out for on the M11/9 are these.
The hockey puck style charging knobs can break off during firing. Mine whacked me in the fore head when it let go.
The wire form stocks can fold up during firing. This is a dangerous condition with the M11/9s but it also happens on the thicker MAC-10 stocks. (SGF, ask Chad about his folding up on him during a mag dump!)
Some 1986 built guns don't have hardened parts. This may be the biggest advantage in buying a used gun as hopefully all the bugs have been worked out. The bolt sear catch notch rounded off on my first M11/9 and resulted in a runaway gun. I've personally witnessed two others have the same problem.Soft sears can also be a problem. I've never had a bit of trouble out of my 2nd 1986 built gun and it's serial number isn't that far away from my first. In fact, it's serial number was higher so there's really no good way to tell.
The selector shaft also serves as the sear axis pin and it takes a lot of pounding. I've never broken one, but it happens occasionally.
I've got a Practical Solutions Mini Uzi style side folder that's rock solid and I use that on my M11/9 when I'm shooting the OEM 1,384 RPM upper. I'd also ditch the front strap in favor of a rigid K-grip to help control the gun in "stock" configuration, but that's me.
I wouldn't order any extra parts thinking they'll break with the exception of extra buffers and maybe a different stock. I would order the MAX-11 upper at the same time you buy the gun. You probably won't shoot the M11/9 in stock configuration all that much after you have the slow fire upper.
I watch the ad boards pretty close and I haven't seen an M11/9 for $2,500 in a long time. That's not to say you won't find one, but anything under 3k is a deal IMHO.
Check Subguns ad board and consider spending five bucks to place a WTB ad.
http://www.subguns.com/classifieds/?db=nfafirearms&category=All+Items+in+this+Category&query=category&search_and_display_db_button=on&results_format=headlines
Sturmgewehr is another board worth watching.
http://www.sturmgewehr.com/webBBS/nfa4sale.cgi
Happy hunting!