I'm not sure what the resolution of that T420 is. It says 1366 x 768 and 1600 x 900. Not sure what Dell to compare it to. I think the dell laptops with 256GB HD have SSD drives. I'd rather have a 256GB SSD than a used 320GB HD.
How about a E6420 for $158 ($289 with 45% off) 4GB and 256GB drive. Upgrade the memory yourself. Both machines have i5-2520M CPUs. After you upgrade the memory it is about $100 less.
http://dellrefurbished.com/product/s...-hdd/si6848541
Or the E6430 for $224 ($409 with 45% off). It has I5-3340, 320GB HD, USB 3.0, only 4GB of memory (upgrade it to 8GB for $17). It has a much faster CPU, USB 3.0 and upgradable to 16GB.
http://dellrefurbished.com/product/s...-hdd/si6829094
It sounds like you have a lot of experience. I have no reason not to trust your opinion. I've had good luck with the my lightly used latitude. My dad treats his Thinkpad like shit and it keeps working.