As Henkm says perhaps don't worry too much about it to start off with. Find a figure you like, clean and assemble and prime it then paint with what you have.
I'm a fan of ancients and medieval myself, but will do anything if it appeals. If you're used to painting citadel sized figures a 54 mm will seem like a giant (They're also a bit cheaper) 75mm is a good size too, but pick something you want to paint and you can afford and then ask questions on the forum you'll get plenty of help.
(Don't rule out resin figures - the quality is excellent and they might be a bit easier to handle)
As to suppliers I regularly use Historex Agents, El Greco and SK Miniatures and would recommend all of them without hesitation. Have a browse on their websites to see what appeals to you (they also sell paints etc so you can get yourself sorted with anything else you might need)
I've just completed a Teutonic Knight which was 70mm high. Sculpted by Mike Blank in resin and pretty good value (about £32 + postage from SK Miniatures) Not many pieces and a simple paint scheme.. There are a lot of others about though; Pegaso, Andrea and Art Girona all make figures in metal which might 'float your boat'.
Have fun and welcome to Planet Figure
Paul
ps - I use acrylics these days, but have used oils and enamels. It comes down to personal choice, there are great artists that use all of them. Play about and see what you feel comfortable with is the best advice. In general acrylics are applied with thin watered down coats to build up shadows and highlights, oils are worked wet in wet to bend the two together because they take longer to dry so they give you more time to work with. That said there are ways of making acrylics dry slower and give oil effects and you can speed up drying times for oils if you really wanted to.