Suppressors are still being made. As a matter of fact it is a growing market. As with anything else, you get what you pay for. There are suppressors out there for $100 (plus stamp) but they are nearly worthless usage-wise. If you stick with some of the brand names, AWC, John's Guns, KAC, Gemtech and many others you should do well. Talk to the mfgrs and see what they suggest as suppressors are like rifles, cars etc. Different circumstances require a different tool (in this case a different suppressor). There are different styles of suppressors and the mfgr should be able to tell you whether you need a baffle system, wipes system, cone system or whatever.
On the length of the rifle, I believe it would have to have a longer than 16" barrel to stay legal or else you would have to get a short barrel rifle stamp also. I believe also that this would be a two stamp project (with short barrel) though your best bet would be to get an asnwer in writing from the Technical Branch in Wash DC. Field offices have been known to be contradictory and may inadvertently give erroneous information. Even though the field office may give you some information you would still be liable for any illegalities that might occur due to the field office recommendation. It has happened enough across the country that it has become a rather vulgar joke.
As to the post-ban semi-auto; my understanding is it still has to conform to all applicable laws (Calif. assault rifle, US 922, etc) so if it already has one "evil" feature then it can't have a suppressor put on. This area has been under discussion for quite a while as it appears to be a somewhat grey area of the law, depending upon who you talk to. Again your best bet would be to get it in writing from the Technology Branch.
As for building a modern DeLisle, sounds cool. There is a company building these with suppressors and with dummy suppressors also. Www.valkyriearms.com