It looks really clean in these pictures - and the paint on the handwheel still looks good - normally, the paint is chipped off there.
Does it have a full kit of attachments? If not, you can pick up slant shank sets pretty cheap on eBay. You just buy a set that's intended for the early Touch & Sews - those can sell for $5 or $10 because there isn't much demand for them - but the 503 takes the same needle plate, cams and feet as the early 600 series.
It takes an ordinary 15X1 needle and a class 66 bobbin. It's not very good for free-motion work, but you can use it for darning type repairs and even free-motion if you don't care about the appearance on the back of the piece.
The mechanical buttonholer makes beautiful buttonholes and the machine can take on most household sewing tasks easily. It doesn't handle heavy loads - you will often get broken needles if you try to sew over multiple seam allowances in heavy fabric like jeans or twill. Better to use a vertical needle for heavy stuff like that.
These are still very cool machines and they sew beautifully. :)
I have the manual I can send you by email if you'd like it.