Ok, $1k for a JD400 is a steal since you say it is in prime shape with a 3 pt and the deck. You can't go wrong.
The front/rear/power steering are all on the same hydraulic system. So if the front ports are not leaking then you should be fine.
The front PTO is a 2000 RPM PTO. The mid-PTO (where the mower deck shaft hooks up is a 2K RPM PTO as well. It's really the back side of the front PTO so...
The Rear PTO is really a gear reducer. It bolts up to the rear end and has a drive shaft that runs to the Mid-PTO. It takes the 2000 RPM's and reduces it to 840 RPM NOT 800. That is really a bummer because it kills all chances of using a rear mount mower unless you are brave enough to overspeed one meant to run at 540 RPM.
If you want to run a tiller for the 400, you will need the Rear PTO ($350-450, anything else is too expensive) and a 35 or 35A tiller (I have no idea what the difference is) The tillers generally sell in the $800-1000 range and only pop up occasionally on Ebay but they do show up.
You can mount a 4 way 54 inch front blade on front blade. They are in the $400-500 range, IIRC. You can get a blower for a 400 as well. It is hydraulically controlled as well. I don't track prices on those but generally $500 and up gets in the range. Both blades and decks show on a regular basis on Ebay and other places. You won't find them much on CL unless you happen to live in the Mid-West/North Central US. So you maybe in luck.
The 20 Loader is a bit of a rare piece but the St. Louis, MO CL has one for sale right now for $1500 but it needs some work. The frame bolts right onto the tractor, but you will need a couple hours and a buddy to do it the first time, ask me how I know. If you mount the loader, you can't mount a mower deck, only rear attachments so....make sure you have a need for a FEL or have a good back up mower.
There are a bunch of other attachments you can find for the 400 but they tend to be a little hard to find. Most attachments are indeed 400 specific but the most common ones folks need for mowing and snow removal are pretty easy to source.
I would buy it for $1k and if you don't like it, you can sell it for twice that on Ebay without too much sweat. Wait until the fall or next spring to do it though.