An A2 board used costs you about 1k USD (way more new). Then, if you are luck you can repair all boards (which I strongly doubt). If you need another board, the used price is someweher between a few hundred to 1k, usually, so make your math. So best guess is, having repaired and calibrated many 3458A's, you end up at 4k, if you are lucky, or more. Keep in mind, many parts are not obtainable, or expensive, so you in that case may need a new board, unless you have spares somewhere. You should also ask yourself why no internal pictures are available. Obviously, they opened this unit up and could have made them (or make them on your request). This may also be a unit that has been 'complied' from various non working ones. That would even be another level of fun...
I have read the D4910 story from this seller on eevblog, and my conclusion as far as they are concerned, is pretty clear.
This in no way is repairable economically. You are better off buying a new, test one at this price with CAL.