If you are on a tight budget and the jointer and motor are in good working condition, save for new knives, fence and guard, I would think it a good deal. As others have mentioned, make sure the beds are in good condition. Individually the beds should be flat to within a few thousandths of an inch and should have no twist over their length. They don't have to be perfect relative to one another -- you can correct those [small] errors with shims -- but they must be perfect individually. You can also shim the cutter head, if necessary, to bring it inline over the length of the outfeed bed. Make sure none of the gib nuts are not stripped or missing in the cutter head and that the cutterhead is complete (you may need penetrating oil to remove the bolts to loosen the knives -- who knows when the knives were last changed). As others have stated, make sure the cutterhead and motor turn fairly freely (expect some resistance from the induction motor, but not alot).
You can easily build a makeshift fence out of 3/4" MDF (just lookup homemade bandsaw fences) and clamp or bolt it to the jointer (make sure you have the rear 'table' that the fence would have attached to, this will give you a good place to clamp or bolt the fence to). Truthfully, you will almost always use your jointer with the fence set at 90deg, so I don't consider the homemade fence to be a big deal. If you mount your fence and find you are off just a bit from a perfect 90deg, use some brass shims to adjust the angle until perfect, then clamp or bolt in place.
As others have stated, a makeshift guard isn't terribly difficult to make either. Your guard wont include a spring to automatically open and close like a factory original, but you will be able to set your homemade guard to close off the portion of the knives that exceed the width of your jointed board, which provides you with much of the safety of the original (just be sure to set it to fully cover the knives when not in use).
Plan on giving the beds a thorough polishing, the smoother they are the more easily you will be able to feed your board through. Invest in some good push paddles to keep your hands out of harms way when face jointing. If you use the foam-padded push paddles like I do, cut a bit of foam off the rear of one of the paddles and screw in some 5/16" to 1/2" square dowel to the rear of the paddle. This will give you one paddle that hooks the backend of the board and allows you to feed the board through without the pad slipping.
Most importantly, at $50, even if things go south with the purchase, at least you can make up a portion of that by recycling the metal!
PS -- make sure you have a good straightedge for your evaluation -- you would be surprised how much error a typical I-beam level can have over its length if you try to use it for a straightedge. Your straightedge should be at least 2ft to inspect each bed, but you will want a good 3-4ft straightedge to bring the two beds even with one another over their lengths. Some of the new box-beam levels with factory ground sides can make for decent straightedges (if at the hardware store, grab two such levels, put their ground faces togeter -- if you see light they are not flat, if you do not see light then they are either ground flat or have perfect complimentary errors -- try a 3rd or 4th to verify the errors are not complimentary). Expect to pay at least $40-60+ for such a 4ft level (which is still much cheaper than a precision machinist's straightedge -- which is the ideal you are wanting to approximate).
You might also wish to see if any of us are in your area and can help you look over your setup once you think you have everything setup right (and before first use). An experienced user will be able to help make sure your beds are planar, knives setup properly, etc. and share with you proper technique for safe jointing. Most folk on NCWW are more than willing to lend their expertise, so please don't hesitate to ask when in doubt.
HTH
