Go to uk pinball league events, and go to more shows to build your knowledge base. UK league has a mix of abilities in the "league" format which is regionally based and you get to play about 8 games at each event
http://ukpinballleague.co.uk
http://ukpinballleague.co.uk/uk-series.php The ukcs is a separate series of knock-out tournaments
http://www.northeastretrogaming.com is a show up north
Get to know folk you meet. Best way to buy a pin is from a known entity on this forum/ the uk pinball league crowd, and definitely NOT an unknown geezer on eBay. Buy from someone who loved the game, kept on top of maintenance, rebuilt worn components ...... Idiots on this forum like me take a "working" game that a casual observer would think is fine to bits, clean and replace any worn moving parts, clean all the switches, rebuild all the flippers with new parts, but plastics sets to replace cracked ones, buy new ramps ...... making the game much sharper to play
Everyone on here looks at eBay, but buyer be very beware. Plenty of folk out there selling tired, worn out games that they wipe down with a rag and say is "refurbished". Guys who just own a single pinball, been in the garage for 10 years, never been maintained, stopped working properly ...... then flog it on eBay. Look at a "shop log" on here to see examples of what refurbishment really means
http://www.pinballinfo.com/community/threads/fireball-2.27480/
Pinballs vary enormously in cosmetic condition, desirability of title, playing condition. Some games you gel with, some you don't. Some "game specific" parts - the cosmetic stuff like ramps, plastic artwork, backglasses are expensive/ hard to find/ require import from usa
You also need a view on how many games you will end up with. I had my addams family 15 years before I stumbled across the uk league and events this year and now have 8
If you have one game only, it needs lastability/ complexity to keep you interested. If you have a dozen games, you can include a few simpler (and cheaper) titles you play less often
As a first game, i would suggest you get a late 80s williams system 11 or early 90s wpc game. These have in built diagnostics that help you fault find. Plenty of folk will buy it from you if you buy wisely and should you decide to sell. So you might buy it for 700 to 1700 and get about that back when you sell it.
Buy a working game as a first one, if you buy a wreck, the initial refurb might be too much for you. Think about refurbs when you are more experienced