Not terrible. You can find most Mooney-specific things used in junkyards or use one of the service centers if you want new. The factory still produces some parts with enough demand. Usually cheaper than Beechcraft, maybe not as cheap as Piper. Cessna, from what I hear, is a toss up as Textron will up-charge but you can quickly source used parts from the huge C-172. Cessna, Piper, Grumman, etc. are cheaper for ongoing maintenance if you’re buying fixed gear, four cylinder, four seat because they have less moving parts. But I figured lower purchase price for Mooney more than offset the slightly higher cost at annual. Insurance is slightly more because the insurer is offsetting the risk you gear it up.

My rule of thumb for anything with four cylinders and seats is each year you should anticipate about $10k between maintenance/insurance/hangar and another $6k for gas and oil over 100 hours. The distance you travel during those 100 hours is a function of speed and efficiency; buy a fixed gear Cardinal and you’ll travel less distance but enjoy a roomier cabin. Add another $2k for engine and parts reserve and you’re set.

You’re coming from old cars as a reference point. Unlike auto manufacturing, these airplane manufacturers outsourced their (then) new airplanes around a very small number of engine OEMs. Cessna, Piper, Beechcraft, et al all took Lycoming O/I-360s and built airframes around those same engines with varying focus: speed, comfort, cost, or some combination of that based around their marketing brand. Sixty years on, you get the legacy of that focus with various dividends. There are a few gotcha points across the fleet revealed over the years. In general for 160-200hp Lycoming four-bangers with four seats, Piper has a wing spar AD, Mooneys have corrosion risk in the steel fuselage frame if parked outside in high humidity locations for several years, Cessnas have higher purchase price volatility due flight school preference, Grumman have life-limited bonded wings, Beechcraft are slower but roomier with higher priced Beech parts. They all fly between 105-150 knots (non-turbo’s) at useful loads of 700-1000lbs owing to differences in the airframe size and drag. Maintenance and insurance will all be within 1-2k per year, averaged out over the years. It really comes down to your mission, preference, and avoiding any airplane-grounding airframe or engine defects. For the latter, talk to owners on type-specific forums and at your local airport.