Average guy can do it, if you have the drive to do it, it's not that bad. Sounds like you already know Clay has the best prices on them with great customer service, here is a link to Robin's writeup and he was not using Clay's newest tool IIRC Robin's Injector Cup R&R

i would send the injectors off to have them flow tested while you had them out and if you could do without for a few days, Riffraff Diesel: HEUI Flow testing

GP's are cheap enough that while you were going to be in there I would change them all personally. But you could ohm them out to verify proper resistance.