1) Tractor (needs to be 4WD as it is a little hilly). Was looking at a 17hp Kubota- too small too pull a baler? (something tells me I should be doubling this?). Also how easy is it to increase width of tractors to increase stability?
17hp kubota wont be very stable on a hill, as you suspect, too narrow, and too light, and also wont pull a baler (nor the hay cart once you have baled and need to get it in)
You could look for a non vintage (overpriced) but old machine, nice and simple to repair. eg we got a 1974 John Deere 2030 75hp which does smal bale haymaking very nicely (NB this is not 4WD altho there are similar aged tractors which are). Ours came from eBay, restored, with a haybob, and a couple of carts. If the ground is really steep, look for an 'alpine' tractor which are 4x4 but also low slung. Not cheap tho!
2) Finger bar mower?
For 7 acres I would go for a conventional mower, they arent expensive second hand (nb you want a grass/hay mower not a topper, flail mower or mower conditioner (the last injects the grass back into the ground to fertilise it which isnt quite the idea when you want to make hay :-) and the first two wont cut low down enough and will mash up the structure of the grass)
3) Hay maker/tedder/woofler/windrow?
Agree look for a PZ haybob or similar. We got one, and altho parts arent readily available off the shelf, we later got another and the first one provides spares. Usually just the odd tine.
4) Conventional baler?
The New Holland 276 superhayliner is what we use. All of the ancient small balers (they are hard to get new as most farmers do big bales now) are temperamental but they all work with a bit of love and a degree in string tensioning :-))
Dont discount eBay, if you pay when you collect the item not on purchase then you do get to inspect the machine and we have got nearly all our equipment that route.
Re: the hay, it might look quite green and meadow-y but the sheep absolutely go mad for it and will refuse to eat any bought in stuff in preference.......we make about 150 bales just cos thats all that will fit in our barn....
Here are a couple of piccies to whet your appetite for all things hay :-)))