You probably won't go far wrong with Williams/Bally titles from the 90s. There's an excellent supply of spares and many are so popular that game-specific parts, like plastics, are being reproduced. Also, the system itself isn't overly complex so repairs are relatively cheap/easy. On the flip side this popularity makes them desirable and therefore expensive (what pin isn't expensive these days though?).
Outside of this things get a little different. More recent games are generally well built and probably won't have a history of being hammered on-site, but this will be reflected in the price tag. But the electronics in the control systems can be very complex and is generally not repairable except by the manufacturer or experts. It's more often the case that a board swap-out is the simplest, but most expensive, option.
Older games from the solid-state era of the 70s and 80s are cheaper as generally less desirable because they're not as complex/interesting and have probably had a rough life over the last 30 to 40 years. Some though are extremely collectible, e.g., CENTAUR, FLASH GORDON, FATHOM, and can command prices as high as any DMD. However, there are still bargains to be had and if you like tinkering with tools and a soldering iron they are easy to fix as the electronics are very simple by today's standards. The hardware is generally readily available as spares, e.g., coils, flipper mechs, etc., but it can be very hard, if not impossible, to find spare plastics, backglasses or playfields, as very few, or no reproductions are made. Some parts are just "unobtanium" and would need to be custom made (I did this for a unique chrome metal plate on my Bally GOLD BALL - got an engineering friend to cut it on his CNC machine).
You can of course go further afield into either older games or the EM-era, or look to lesser known manufacturers. Similar trends apply. EMs are mind-bogglingly complicated and spares are notoriously difficult to find. Older games are also much, much simpler and unlikely to appeal unless you remember them from your youth. Lesser known manufacturers, such as Zaccaria or Data East, have made some very cool games, but again, parts will be harder to find and possibly more expensive.
Bottom line though, pinball isn't a cheap hobby. It doesn't have to be an expensive hobby, but it's definitely not cheap. My personal preference for unloved Bally solid-state games means I probably operate at the very bottom end of the hobby. My cheapest pin was £125 that I bought for spares but realised I could save (a very undesirable Bally GOLD BALL). I've probably spent that again on spare parts and have probably put in at least 200-300 hours into its restoration (see the
shop log) including completely rebuilding the power supply and repairing the MPU (motherboard) including tracing digital logic with an oscilloscope (hey! it's my thing!).
Best thing though is being a member on here. Every one is super helpful. From the players who know rulesets inside out, to the collectors who know the history (sometimes down to individual machines), to the tinkerers servicing their own machines, to the restorers bringing old games back to life, to the engineers like
@myPinballs creating upgrades, mods and even software re-writes. Between us there isn't anything we can't do or fix!