The MD7a is not a bad engine for its vintage but they are all getting pretty old. Some things to think about:
Very few of these engines have fresh water cooling (usually an owner conversion) and raw water cooling has its goods and bads. It is simpler but there tends to be a lot of buildup in the cooling passages. Every ~10 years, it is a good idea to flush the cooling system with Rhydlyme or Salt Away or one of those products. Removing the thermostat allows some access to the passages also.
Parts for this engine are relatively expensive but in my experience, you need very few of them. In the grand scheme of things, I don't think that this is a huge issue unless you get one where just about everything is worn out. Some of the parts are pretty weird, especially the gaskets for the cooling and fuel system.
Access on this engine can be good or bad depending on the installation. The raw water pump is on the back of the engine and the alternator is on the front. Whether this is good or bad depends on the boat.
This engine is definitely noisy. It is a 2 cylinder with a giant flywheel so the vibration is definitely noticeable but not horendous. The engine itself is noisy and the exhaust is very noisy for an engine this size although you can muffle the exhaust some.
Many of these engines are getting old and have poor compression which can for long cranking times when it is cold. My experience is that cold cranking times are something like 3s for >70F, 6s for >50F, 10s for >40F on an engine with decent compression. The starters are definitely oversized for these engines so as long as you have the batteries, you can crank them a lot.
Given a choice, I would go for something newer but I wouldn't consider one of these a complete deal breaker. While no one ever plans for it or wants to pay for it, if the worst did happen, a Beta marine engine will fit in place of one of these with a little engine bed modification. What type of boat is it in?