The search here is not functioning, try a search on Full-auto.com for special weapons speshul weapuns, todd, toad & you'll see enough horror stories to make your hair stand on end. Some people seem to have gotten some okay stuff from special weapons, but not many.
I think the Cetme's sound like they are also hit and miss, but there do seem to be a lot of guys here who have them and are happy with them. I don't know about modifying a Cetme, there are some good Cetme boards, but I don't have the links handy. What about a DSA FAL? Are those in your price range? I know they're not HKs but they sound like good guns rather than "gamble guns." Springfield Armory made HK 91 clones for a while, they seem to run in the $ 1000 range used. I don't know anything about them really (built on greek receivers maybe???).
In an AR v. Cetme or Speshul Weapons contest I'd pick an AR. This might be more true in your case because it sounds like you like to tinker with the gun so you can swap uppers etc with the AR.