For a comparison, about 2 years ago, i picked up a 907 5' (22-123) for $100, figuring if i found major issues with it and not knowing parts availability i could recoup that as scrap iron.
After a careful inspection, i found it needed gearbox seals, an output shaft bearing on the cross drive, and roller bearings so with the needed parts i've got around $200 in it plus a belt replacement and a couple of other incidentals.
According to the ford/new holland website, the 907 you mention was built from 1964 to 1972 so parts availability particularly from fnh is probably non existent.
Things like bearings, gearbox seals, the common stuff will be available at a good bearing supply, and cutters are available from flailmaster, BUT any model specific parts you may be out of luck should they require replacement.
A word of caution here, after doing alot of google time, reading, research, and so on, a weak point of the 907 seems to be the (lower) driven pully, as i've seen several requests for that part on various farm forums, that have gone unanswered.
The problem that lies with locating that pully or a substitute is the driven shaft is an oddball 1 3/8" 10 spline and that seems to be non existant.
Thinking to the future, i have found a supplier that stocks a 1 3/8" 10 spline shaft coupler, weld-it hub and pulley, though would require some machining and welding the whole assembly together should the bottom pulley fail.
As to 22 hp being enough for a 6' flail, i'd say marginal. A 22 hp class tractor is probably in the neighborhood of 16 or 17 pto hp, so may be short of power in tall grass/tough cutting.
Another concern may be your tractor's 3 point itself, be sure to check the lift rating on the arms.
you can look the 907 up on the fnh website:
New Holland USA - Tractors - Agricultural machinery - Combines, etc.
go to the parts and service link at the top of the page and then click search for parts in the drop down menu. type in 907 in the search box and locate the 22-124 model for parts reference.
Considering All the above, unless the mower is in top notch shape and low hours, (which isnt likely) i'm thinking $625 is pretty high. Depending on what if any repairs needed, i'd only be willing to pay $200/$250 range myself.
If you'd like, feel free to pm me, and i'll gladly answer any questions i can regarding the 907.