Get a 1600 series but the R version which has a bit more horse(bit?) power. You want 8 meg of Flash.. at least 8 meg of RAM. THe ones on eBay tend to have 4meg Flashcards but it's a very simple upgrade for about 50$. The intergrated WIC-T1 is a clean way to hook up the T1, no external DSU to muck around with.
You can get the eBay router on a service contract with Cisco.. I've done this several times without a problem. You did not say if this is for internet access or just a remote site. That will drive the IOS level needed and in reality the requirements of the flashcard/DRAM. I run several 1600s with T1s on 4meg FLASH and 4 meg DRAM but then they never come close to using the full T1. And if you want to start using something like OSPF or EIGRP, that drives up the DRAM requirements. My 8 and 8 recommendation allows plenty of room to grow and enhance the router *if* needed.
Just my 2.5 cents worth ;-)
Mike S
"Diplomacy; the art of saying 'nice doggie' till you can find a rock" Wynn Catlin