[Freesail99]

I think 15 to 20% of your budget for refitting and provisioning is a bit lite. Spares or spare parts that you should carry will take a large part of your budget. Systems that you need to replace may surprise you at there cost, even doing much of the work yourself.
As far as your question if a 20 to 30 year old boat is to old, it really depends on the boat or how it was cared for. Certain makes of boats were also more prone to blisters.