so I have been looking at a 40+ year old Ford 907. It's in OK shape but I'm worried about parts availability (and cost) and it only has 33 knives across a 72 inch rotor. Means a knife every 2 inches or so. Any advice on how good a unit it is and will it do what I need it to do?
Having used a 907 for several years, what i've found is overall they are pretty trouble free. Based on personal experience, a couple of observations,
Check the drive belt regularly, being a single belt drive, tension is critical. If the belt stretches a little they are prone to slippage.
Model specific parts for the most part are not available from FNH, but the mowers are built like a tank. The only major part on them that one would have to worry about replacement not being available is the rotor, should it get bent. Even the rear roller could easily be fabbed in most home shops.
Cutter replacement is a bit of a pain, instead of each knife being mounted on a seperate stirrup, each row of knives is mounted on a common rod threaded through the drum via a hanger/stirrup arrangement and the rod has to slide out the end opposite the drive belts. It takes a couple of hours to change them out, but they are easily sharpened.
Check the model number on the back of the mower to the right and a little below the gear box, there will be a number such as 22-xxx. The 6 foot models were either 22-108, built from about 1957 to 1979, 22-124 built from 64-72, and a 907 without the 22-xxx model number was built from 62-72. (per FNH website)
FNH website also shows that dethatching knives were available, though like cutters will probably only be available aftermarket, from someone such as flailmaster. Rough cut and finish cut knives are also available though the flail won't have the capability of cutting brush as heavy as a bush hog type cutter can handle.
Finish cut knives do a decent job though not the same quality cut as a finish mower, you also dont have the windrowing of cut grass behind the mower that a rotary cutter can give.
Bottom line really is the purchase price vs any repairs needed. As an example when i picked up my 5 ft 907 several years ago, i replaced gearbox seals, one bearing on the cross drive, and the belt. Since then i've replaced the roller bearings and cutters for a total investment of around $300, including the cost of the mower.
If the mower is in decent operating condition, mowing 3 acres you should get many years of service, and if you can purchase it at a fair price, should a non repairable break down eventually occur, with scrap iron prices being what they are, you'll recover most of the purchase price.