A couple of things you need ask yourself first:
Do you want to work on the machine yourself or are you looking to collect machines and have them serviced/repaired when needed?
Are you looking for a machine that's fully restored or do you want to bring it back yourself?
What's your budget?
Is this a one off or the start of something big? (Once you own one, the second is not far behind.)
If you're looking for a fully restored machine and money is no object then Henry Powell is ...an option, that is what HP sells, but it won't come cheap and don't expect much in after sales help. You can however rely on all the help you need here on this forum.
The "early" machines (1930s to 1950s) Jennings, Mills, Buckley, Pace etc, are all well made and pretty much always repairable if something goes wrong. These would be my choice, but you would need to be prepared to work on them from time to time to keep them up to scratch.They would not be cheap. If you bought a Jennings Chief you would probably keep it forever.
The "later" machines (late '50s to late '60s) Aristocrat, Jubilee, Sega etc, are pretty much bomb proof and just need to be kept clean and well used to go on forever. A nice Aristocrat Nevada might be a nice looking machine to start with.
The "modern era" bandits (Bally, etc) are moving into the electric age (although only lighting on the first ones) so are at the end of our remit on this forum. Nothing wrong with them, but not the classics of the earlier days. An early Bally "Money Honey" series machine might work if you like that sort of machine.
German bandits and Uk machines with German mechs you must beware, nothing wrong with them. I have several myself, but prone to parts breaking due to the poor quality metal used and the fact they were built to last just three years and are now 70+ years old, so be prepared to do work on these.
As to buying online... it's always a risk of course. If you see a machine you're interested in, ask the forum what you need to know about it. One of us may well be in the area and able to look at it for you. If you can wait till March the next of the great Coventry auctions that are closely associated with this forum will take place on the 24th. There WILL be a machine there to suit you.
Also be aware, one machine is never enough.