My only recommendation would be to buy it sim-free, so you've not got the T-mobile variant firmware, then put your Tmobile sim in it... That's likely to work out a lot more expensive though.
The N70's a wicked phone, I love it, and so do most others... there are some problems, that you'll be able to find out about by reading on here, but nothing to put you off getting it.
Nokia History: 3110, 5110, 7110, 7110, 3510i, 6210, 6310i, 5210, 6100, 6610, 7250, 7250i, 6650, 6230, 6230i, 6260, N70, N70, 5300, N95, N95, E71, E72
Current Phone: HTC Sensation - Android 2.3