All Android devices do not require a data plan in order to "activate". During the initial setup, many manufacturers conveniently place a setting in there to disable 3G/HSDPA+/LTE connections so that users without a data plan do not get charged with ridiculous prices. With that gone over with, it definitely indicates that you'd be able to get practically any unlocked device off of eBay and get it to work on T-Mobile.
That being said, T-Mobile (as well as AT&T) run on GSM, as opposed to the CDMA networks ran by all other major US carriers. This puts you at a limitation, as CDMA devices are never compatible with GSM networks. This will promptly cross off all the devices that have come from Verizon, and other US carriers. You're likely going to have to import from an international market for a broader range of phones, or opt for a third-party sold phone from either AT&T or T-Mobile themselves.
It also should be noted that T-Mobile uses an unusual frequency combination for their 3G data network (1700 and 2100), meaning that devices that are compatible with their data network (once you do opt for a data plan) are quite limited. That being said, however, their 2G network runs on a very standard set of frequencies, and thus shouldn't affect you much in picking out a device, as you're not utilizing their 3G data.
With that out of the way, there are plenty of devices to recommend purchasing. If opting for something from T-Mobile themselves, I'd go with their new T-Mobile MyTouch 4G Slide from HTC. Fantastic device from what I've heard.