New? Used? Price? What type/length of buttstock?

OK, Jorge, nice to see another Rhode Islander here.
My first suggestion would be to survey the other makers of ARs since the equivalent rifle from another quality mfr might be as well made or better, and very likely considerably less in price.
Because of your stature, the buttstock length is the issue.  The older A1-style buttstock is shorter than the A2-style, and also has the cleaning kit trapdoor buttplate.  The trick is to obtain both of the shorter A1-style buttplate screws, which are identical to the A2 screws, ecxept in length.  You could order A2 screws, and carefully trim them to length.
A1 stocks w/buttplate can be had at [url]www.reesesurplus.com[/url]
Coincidentally, I'll be adding an A1 stock to my AR shortly (no pun intended)  Good luck!

No sweat, Jorge.
Well, Bushmaster, Armalite, DPMS, and Colt are the biggies, but there are other, smaller mfrs like Rock River and others that make a good product.
Does a Colt have an enhanced collector's value?
YES, ONLY if the buyer thinks it does.  Collector's value also greatly depends on condition, so a used Colt may be worth less than a NIB Armalite, all other things being equal.
You might want to consider a Bushmaster Dissipator.  It's avail w/wo handgrip, and with either a heavy bbl (std), or in M4 (lite) bbl config.  The thing I like about the Diss. is that it retains the original (A2) sight radius, so that it is a little more accurate, and the graduations on the sights all work properly.
Also, BM bbls and chambers are chromed, an asset in salt-air environments.