I'm a sucker for old iron that just needs a little luv. That seems like a good candidate, in that it has nice features (back gear, power quill, table feeds). It is certainly an ambitious project. I have taken on some challenges, but mine have still been in one piece. You are correct, that a little rust isn't that big a deal (as others have states, that actual ways are pretty tough). It looks like you already have a small mill, so this makes a good compliment. I was in the same situation, with a small mill (a Maho) and kept my eye open for a larger mill (which ended up being a Cinci Toolmaster).
That you offered him $400 was generous on your part. If all is good, there is probably a month of hard steady work - much more depending on components broken, missing, worn out, . . . . .. In my world, that would take several years to complete.
Attached are a couple of my projects:
- A good sized drill press for $100, plus another $30 for parts and a tour through my excess tool storage (like most here I don't hesitate to pick up tooling that I can't use - when it is really cheap). I spent about 60 hours, cleaning, making components, repairing. It was said to have a "bent spindle". I don't know how that would be possible with the machine still being in one piece! Anyway, I replaced the chuck/arbor and all was good.
- The Cinci Toolmaster 1D (is probably on par with that Index - does not have the nod feature, but has the Reeves drive on the spindle - otherwise very similar). That transaction took about a year and a half (patience required), then I had to collect it immediately (my guess is that other buyers backed out). Anyway, it had been in cold storage. It was very dirty, a bit of rust (nothing like your project), no option to try it - totally as is. There was only a vise and a MT4x40 adapter with a 1.25" drill bit (there was no z-axis crank or spindle wrench). There were a few broken knobs and bent handles. It was/is a well featured machine. The rest of the story is that it has cleaned up well, have sorted out most of the handles & knobs, the vise turned out to be a 6" Kurt - it is a nice machine to use. I have about 100 hours of cleaning and making parts invested. I've since spent about $400 on basic spindle tooling. The macine itself? $700.
That Tree is a worthy project, but there are more out there. Continue getting a feel for machines available to you. There is nothing wrong with letting them get away - you never know what may happen. I had the chance to buy the Cinci a year earlier, for probably 3x the price. I let it go. Then a year later the opportunity came up again. Don't sweat it. See if you can stay in touch with the seller. At $900, I doubt it will be going any where (then one day he'll call the scrap dealer). In practical terms the market for a rusty machine in pieces is extremely small.
Let us know how your search goes.