As Neil mentioned, the boat your dealing with and what your plans are for her plays into whether you do a $10K diesel swap or a $2K A4 rebuild. In most cases, a $10K diesel in a boat worth $5K doesn't make sense.
If you do decide to rebuild the A4, I can attest to how simple and gratifying that is. Here's my thread from the rebuild I did last year:
http://www.moyermarineforum.com/foru...ad.php?t=10566
Total cost was around $1500, including Moyer parts and machine shop work. I'm a mediocre (at best) back yard mechanic with minimal tools, and with some guidance from Moyer and this forum I was able to pull it off.