Max,
Unless the price is pretty good, I would look for a small pivot pin. The advantage with the small pivot uppers is that you can buy just about any upper and mount it on your lower and go. Just the same, there are adapter pins out there (I didn't like them as I owned a Colt R6600 years ago) that will alow you to use small pivot uppers. The small trigger pins are nice as you will have lots of trigger options and unblocked is something that a lot of people are after as who knows when you will want to commit a fellony and spend 25-yrs in jail (after making an LL or DIAS).
[Note: If you buy a rLL or rDIAS just buy a new lower and save the hastle as anybody that can afford a rLL or rDIAS can afford another lower].
Personally, I like to look for lowers that have the raised fence that is drilled for the mil-spec-pivot pin. If it is not drilled, but it has the raised area, you can have any decent gunsmith drill the hole for about $20-$30. This will allow you to swap uppers without a screwdriver as this screwdriver dependance is a Colt oddity!
Prices on pre-bans have been dropping and my favorite of all the pre-ban blue label colts is the Match HBAR. I think I have owned about six of these since about 1996 and I have one in my own safe now. Even with the Match HBARs you will find huge differences so you always need to ask for the specifics and look at the gun before you spend your money...
Questions you need to ask and consider include the following: Large or Small Pivot? If Small pivot, 1 or 2 screw? If Small, raised fence where takedown spring and detent would go? If raised, is it drilled? Large or Small Trigger pins? Blocked or unblocked lower?
From there, and if it matters to you, you can look at the various styles of sear block Colt has used over the years? You may ask to confirm if there is a block (there are a few small pivot unblocked lowers out there). As for describing the differnces between the differnt Colt sear blocks, I know I have seen two or three different styles on guns I have owned but it it is not something that I have tried to study in any great detail.
Also, if you want to run a 22LR or 9mm conversion, you may need to file down the top of your sear block. The block prevents the use of an M-16 style carrier and most 9mm carriers are M-16 profile and I also recall that some people have needed to modify the block when using a ciener kit.
The other thing a block can do is make it difficult if not impossible to use some trigger groups without the removal of the block. The KAC trigger is one trigger system that I thnk requires removal of the sear block to use BUT given that your blocked lower is most likely a large trigger pin lower you will also have a tough time finding a KAC trigger that will work as all KAC 2-stage triggers I have seen are small pin! I solve the trigger situation by using the Jewell 2-stage which works with the sear block and it is a great trigger plus it is available in small and larhe trigger pin sizes!
Finally, confirm that the rifle you are buying is really a Pre-Ban. There are lots of places with serial numbers listed but their are lots of people out there that would love to have you pay pre-ban prices for a post ban they made look like a preban by making an illegal SAW!
Oh well, now what was the question again?