QWERTY feature phones are a dying and rare breed these days, and so you'd probably be better served shopping used than new.
If you care nothing about internet or email access, an old Nokia Symbian s60 handset with 3G GSM 850/1900 support would probably work well.
As for needing an "unlimited" plan,
there is no such thing, and nobody needs unlimited anything anyway.
Figure out what you
actually need (do the math and check usage numbers), shop around, and then pay for what you actually need.
US Mobile is a good option for limited plans with T-Mobile only access with no roaming. However, if you somehow do need talk and text time in excess of 1500 minutes (read 25 hours of talk time) and "unlimited" texts with zero data,
Airvoice Wireless which uses the AT&T network without roaming has an "unlimited" talk and text plan with 100MB of data (useful for sending/receiving MMS messages) for $20 a month, though their definition of "unlimited" is nebulous as it is with everyone. Same deal with
H2O Wireless' EasyGo Wireless down to the network and the nebulous fime print. Figure your abuse threshold these days is around 2000 minutes and around 2500 texts... which is a ridiculous quantity well above national averages for everyone but road warrior truck driver types and sales people who live on the phone.
By the way, all this stuff has already been covered in great detail on the guide
stickied here in the forums (
unabridged here).
