The A4 weighs in at about 450 lb. and a lightweight OB on the stern will cause the bow to be a little deeper in the water and with the stern raised a bit. You may need to rake the mast a bit to fully compensate. Take a good look of how the boat sits in the water with respect to its OEM waterline, especially with a fully loaded bow water tank.
For a relatively inexpensive A4, go to Moyer Marine Atomic 4 Engine Rebuilding and Parts
for a rebuilt/reconditioned A4. Whatever you do dont 'throw away' that fuel tank ... its probably monel and quite valuable even at 'scrap' prices. Ditto, if you have all the solid copper 'exhaust piping' and 'steam riser pipe'. The 'steam riser pipe', etc. is easily rebuilt by a knowledgeable "red-metals" welder.
BTW... forgot to mention - osmotic blisters: Pearsons of this era will have mostly 'cosmetic' osmotic blister and they will rarely if ever penetrate into the matting layer or deeper into the structural layers beneath, only the gelcoat. Unless you see blisters about the size of 25¢ piece I wouldnt be concerned with 'blisters' on a P30, especially on a $3.5K boat.
If you do find blisters and do elect to remedy, most DIY blister repair will be satisfactory and wont 'reoccur' if you dig out the blister, fill/fair, and then apply barrier coat.