"...around 7 inches at 100 yards..." Nope. 3" at 100.
"...the massive price tag..." Cosmoline, what massive price tag? You can get one from the CMP for around $500US or less. That isn't massive. The one you saw was likely a Century re-build. Their QC is iffy at best. Most M-1's will shoot just fine. It mostly depends on the ammo. A rest won't help crappy ammo or a poor shooter. The one, repeat one, you've seen is not representative of ALL American surplus rifles.
"...Lee-Enfield No.3 & 4..." That'd be the No 1 Mk II and the No 4. Mk's I & II. The MK III was used by Commonwealth troops during WW I, the No 4 during WW II. Australia opted not to switch over to the No 4 and continued to use and make the MK III. Milsurp Enfields are notorious for have bad head space. Usually not a big deal with a No 4, but can be with a MK III.
Like geojap says, milsurp rifles can vary to an incredible degree. Some have been properly stored and maintained. Others have not. Anything from Century Arms is suspect for both quality and safety. They have a poor reputation for QC and the stuff they rebuild and sell can be downright dangerous to shoot. They have been known to build rifles from parts bins with no regard to head spacing or the safety of shooting said rifle.
When you buy any surplus rifle, no matter where it came from, you must always, always check the head space BEFORE you shoot it. Especially Lee-Enfields. Do not expect to find cheap ammo for any milsurp except currently used ball. Especially, .30-06. The days of cheap ammo are long gone. Some ammo is nearly impossible to find, is corrosively primed or totally unreliable. So make sure you don't buy some odd ball European rifle that hasn't had ammo made in 60 or more years. Or isn't readily available either commercially or as surplus. Japanese ammo comes to mind and most European calibres.
There are BAR's being made that are semi only and therefore you can buy. Be prepared to pay $4,300USD plus taxes and shipping for one. Any others require Federal permits and you living in a State that allows them. Ditto with the mp44/stg44. Be prepared to dig deep.
You need to decide if you ae looking to shoot these rifles or just collect. The price you pay is different. A shooter will cost less than a non-shooter. Non-shooter being one that is in too good a condition or of sufficent rarity that shooting it will lessen its value. Go here andread all the articles.
http://www.surplusrifle.com/downloads.asp