I got my transmission out, apart into (almost) every piece disassembled on a work bench and it was honestly the most terrifying thing I've ever done to a vehicle. But now that I've got it back together, installed and 15,000 km on it running smooth as silk, I wouldn't hesitate to do another one - swore up and down during it that I'd never do it again tho.
I used a rebuild kit that came with virtually no instructions but was saved by the fact that I bought one of THESE
manuals when i got the kit 'just in case' - about half way through the job i was starting to loose my nerve and getting really stressed. I took the torque converter into a local transmission shop to get it flushed and was lamenting about all the parts I had in the kit and how i had no idea where they went - the guy was a godsend - said bring in the parts and I'll tell you - no charge. In the end he basically told me - yup you're doing fine, you're right those are the right places for those parts, you've got a really good manual here, etc etc - gave a couple pointers on what NOT to touch(all stuff inside the transmission - valve body is pretty straight forward)
So from a DIY'er who was feeling pretty low in self esteem during his project - yes it's definitely within the capability but it could be stressful.
Get another daily driver at least on standby so you don't have to feel rushed (I took almost 2 months doing the drop, rebuild and re-install)
Get a good manual - the one I linked (apparently) is a good one - there's also an 'updated' one available that includes the 4l65 data - not sure which is better
Find a local TRANSMISSION shop and see if they'll sell you the parts and maybe a little encouragement
- failing that you could take in the parts to an auto parts store and drive them nuts with 'I need one of THESE and it goes HERE'
The other thing I did before I started is scoured youtube for how-to videos - some are good some are crap, but they at least give you an idea of what to expect
The job seems really complicated and scary till you've done it. Now that I've written all that, if your problem really is in the valve body - It 'should' be a (comparatively) cheap repair at a tranny shop - most of the money in a transmission job comes from the hours needed to take the sucker out and put it back in. The valve body really is right there after the pan comes off. - go youtubing and see what you think!