That doesn't sound quite right - if there are no signs of oil then how can the auto-elec be saying its the oil that killed the brushes.
I haven't had the vac pump apart, but I'm guessing they are just a vane pump and there will be an oil seal on the shaft that goes into the alternator.
If he's quoted you $300 to rebuild then that should either include sorting the oil leak in the pump as its part of the alternator assy, or telling you you'll need to fix the pump as a separate job. Otherwise you're going to be up for another rebuild in 18 months time if thats whats caused it.
Standard alternator is a Denso 40A external regulator model.
Search on eBay for "4bd1 alternator" and there are plenty of aftermarket ones - most seem to be 70A, with internal regulator.
Haven't bought one myself, as mine was OK with new brushes/bearings.
Steve

