1. They should be free, corporate surplus, etc. I have literally over 100 sitting in storage, that I haven't had time to go through - Unusually I dump 10-15 in a box and bring it with me when I fly overseas. They go for about $100-150 easily.
2. The LCD's really suck, and sure you can get in the moneypit of replacing LCD and more, but it isn't worth it.
3. Reliable? Every laptop nowadays is reliable hardware wise.
4. Safe, good question anon, what is safe. Lenovo stuff, Dell Stuff and Apple stuff are relativelt safe, you can lock down the bios/efi and make it hard for most people outside of state enterprise to break in. They all have TPM, and you can do "secureboot" via EFI and disable the legacy shit - the TPM chip on the t420 is easily physically dumpable though with a buspirate or such. I've done it.
Wifi support cards also are eh, you need to change it out - once again not a big deal but money sink for 10yr old laptop.
Intel Management Engine - yeah, it can be stripped too.
Resale value: Nominal, 5yr old macbooks are going to be steady for year 6-8. Personally I'm on the other side where I buy new macbooks and sell in 3 years for 70-80% of value, but it is different from "reliable." No, this is not saying buy a macbook, but financial side, and support side, it does make it easy.
tl;dr t420 good for it's time, but between a new SSD, max out the ram, replace wifi card if going to go loonix/arch puts you in over $300. If you go w/ a new LCD, yeah it goes overboard quick. Didn't even cover battery, and chinese shit.
Touchpad is also shit.