Nothing is cheaper than a porti-potty. Nothing will be more uncomfortable than a porti-potty and nothing will kill resale value more than a porti-potty.
Sounds like the previous owner stopped doing maintenance and started performing bush fixes rather than spend money.
When I acquire a new to me boat, I immediately go through the sanitation system. Vacuflush is one of my favorites. Easy to over haul and easy to maintain unless some one buried the vacuum pumps in an unreachable location. A typical overhaul would be to buy all new toilet bases. You can find them on the internet for $150. With the vacuum generator I replace the bellows and the duck bill valves. The motors are usually built proof, I wouldn’t replace them but maybe buy a spare. I have never need to replace a motor. The only other item with moving parts is the vacuum switch. They either work or they don’t. They are unfortunately one of the more expensive parts to purchase. In most cases the switch is not the problem, it’s the wiring. Once you have done this it will be years before you will need to do any maintenance.
If you hate vacuflush then by 3 marine elegance toilets. You will need to run electrical but you will be much happier that using a porti-potty.