Hooboy...been there, done looked into that.
Short answer - I figured that to do any kind of decent work (which means quality spindle control for concentric valve seat/guide placement or cuts) I'd be in the $10,000~20,000 range. $5K~10 for a base machine, and God knows what for tooling. A good head fixture for a vertical mill (e.g. Bridgeport or equ) will run $2000. Then carbide pilots, seat cutters, guide tooling, concentrically gauges, guide bore gauge, etc for the head side. Then whatever is required to work the valves - machine, stones, and such. Then if buying used equipment, how much $$$ will go into bringing said equipment into spec.
I have a square column mill that works well for many things but cannot hold the tolerances - in my estimation - necessary for seat/guide work. The nearly mythological little used B-port from an old guys basement doesn't exist in these parts. I could spend $8000 on a good Tiawan based 9x49 and hope it's a good one or 18K+ on a new BP. I know where I can get a nice used - well used - seat/guide Torbin Arp for around $5000 with some used up tooling. Also could get a nice Storm-Vulcan head and block machine for a few $$$$ more with it LOL. Where the hell would I park all that...
I gave up, I could not figure the ROI at anything reasonable doing hotrod type heads in this area. Maybe if I lived in a big Metro location, but not here. I'd do it just for me LOL because who needs an excuse for buying cool tools???! But - it wasn't a level of investment I could make at this stage of life.
Shop Racingjunk - lots of equipment on there. Search Advanced, select Tools and Shop Equipment
http://www.racingjunk.com/search?action=doSearch&is_advance=true&searchString=&new_used=&trade=&categoryId1=1132&categoryId2=&categoryId3=&zipcode=&zipcodeRange=200&country=&state%5B%5D=&priceMin=&priceMax=&sortBy=score&resultsPP=20Looked at the eBay link - I'd figure you'd toss/replace about all the stones and the spring compressors are useless for anything but weak stocker type springs. I'm not sure my good Mac Tools air operated spring compressor will even work on the roller springs for this new 351C build.