How far to strip?

So I've got an old Massey-Ferguson MF155 (it's an export model, as far as I can determine - kinda similar to an MF165 - Perkins AD4.203 engine)... it's been lying outside and quite heavily rusted for a few years, though it runs well when pull-started.

Want to sand-blast it and prime/repaint it.

The question is: how far would you recommend I should strip it, as a minimum?

Already taken bodywork (apart from side panels, which are proving tricky...) and fuel tank, as well as starter motor which needs replacing.

What else to take off - injectors? Injector pump? Water pump? Fuel pump? Should I spray the pipework / fuel lines separately, or when it's on, or not at all?

Do only as much as you can do and still enjoy want you are doing. When it feels like you are working to hard, you are.
SDE

If the tractor is going to be a prized possession then it should be sandblasted until it is completely steel color and then epoxy primed. Any rust left on the metal is a place that will come back to haunt you. If it is very rusted it might be worthwhile to have it professionally sandblasted. With a little home model sandblaster it takes forever to clean the metal like it should be. Any place you have removed parts be sure to completely cover any openings to keep the sand out. I normally fill the holes with something and cover the hole with duct tape.

While you have the gas tank off you might take a flashlight and look inside to see if there is any rust there. If necessary clean the rust out but even if it is clean I would put a fuel tank liner in it. A can of Red-Kote is cheap insurance.

I would dismantle the tractor as much as possible to refinish. If the pipe work was originally painted I would paint it and do it off the tractor. It's nearly impossible to get all the way around it while on the tractor. Usually the paint gets damaged a little putting these parts back on the tractor but that is easily touched up with a small brush.