The MT 6700 is a 20", post-ban, HBAR (Heavy Barrell), with a removable carry handle. It is quite heavy and off-balance to the front. (I know, because this is the first AR-15 I owned).
Colt Match Target Competition HBAR: (shown here with some optics instead of the carry handle):http://www.colt.com/colt/assets/images/a2f3_matchtar.gifwww.colt.com/colt/html/a2f5_matchtarcomphbar.html
I think I paid $825 in Jan 2000.
The MT 6731 is named "Colt Match Target Competition HBAR II":www.colt.com/colt/html/a2f6_matchtarcomphbar2.html
This has a 16" Heavy Bbl with removable carry handle. The picture on this web page is actually a 20" MT 6700.
Changing uppers is a whole lot easier than changing barrels! You can get a second upper for an AR-15 for about $400 if you shop around. If you want a colt or bushmaster upper, then it is more like $600 with the bolt and carrier.
Plenty of post-ban complete rifles can be bought for less than $800. Olympic arms has several for even less, check their web site.
OR, buy a stripped lower receiver (this is the part with the serial # so you will have to go thru an FFL) and buy the rest of the parts. I built my 2nd AR-15 for about $550.
Oly lower $124
Sherluk Kit $425 (16" Fluted HBAR, fixed handle)
Then take it from there.