I went through the postwar locomotives, thinking that is all I would want - all Lionel and one MTH conventional engines.
I should state up front- I am a Lionel guy, and you will get many advising MTH or others. Make your own choice- but know it is expensive to do BOTH.
This over about 3-1/2 years. I started w/ an estimated 280 running feet of Fastrack, two ZW transformers. As far as engines, I began with a few conventional postwar steamers, two LionChief Plus - an RS3 and a Pacific- and all were great fun and terrific.
I tried my first Legacy (F3 called F7, diesel set- powered/non-powered AA) about halfway through. Then a Legacy Steam K4, to several TMCC diesels, even a 2000 Century Club 773 Hudson (TMCC). I personally have evolved from steamers to first-generation diesels. I have been delighted with the early 2000's Lionel Legacy engines- quality AND features.
Through this my control system naturally evolved from conventional, and then to TMCC command control where I am today.
So:
1. Buy used whenever possible. Let somebody else fix the common issues these things seem to have when new. Even under warranty- packing them and shipping them- even at Mfg. prepaid basis - is a PIA (if you get my drift).
2. Many of the big bid-auction house train specialist "online stores" are very good sources, know their stuff and back their products. But estate auction houses- I have had a few issues- first they may not really KNOW trains, or THE train you buy. Many times the estate houses have not and do not possess the ability to even do basic "does it run?" types of testing. Second, they many times cost more by the time you pay the buyers' premium, taxes, and shipping. The people here are (in my experience) reliable sources of used stuff.
Never buy anything that the seller has not tested!! Yes you will get a bargain sometimes, but more often there is something wrong and troubleshooting it isn't worth it in the long run. All in my opinion, of course.
3. Buy a ZW 270-watt restored transformer as a power source, with 4 outputs, they work great and are just plain cool. But then add the aftermarket circuit breakers and diodes to protect to protect the newer (post 1995) locomotives from track shorts and from electrical surges.
4. Buy either a TMCC command control system (about $ 100 if you shop bid auction sources) or a Legacy System (a lot more but not a deal breaker, either).
5. Buy from Local Hobby Shops when possible. The LHS generally doesn't have the space or money to stock much in the way of engines- but they DO have structure kits, freight cars, and all those small things we all need. You will need strips of wood, paints, glues, etc. etc. etc.- and its impractical to buy all that online. The LHS's are your lifeline as you get further into the hobby. I never go into one without buying SOMETHING.
Not that you can't buy small stuff online. But someday when you need something fast- you might drive up to an empty building where the LHS used to be. Make yer own balsa, bucko!
6. ANY good source of information, in any scale is applicable. Scenery techniques used in HO are fully applicable in O scale as well.
And- hook up with others- we (as one example) have a great group of 20 guys or so in Northern Ohio who get together to learn from each other.