I had just gotten out of HO (because I was having trouble dealing with the smallness of it) and while surfing the internet one day I decided to see what was available in O scale for Seaboard Air Line.
I spotted a MTH Railking RS3 and ordered it, just to have it displayed on a shelf. Soon I spotted rolling stock in SAL livery, which really started the ball rolling. I've been pretty good at not buying the 1st thing I lay my eyes on which has kept the cost down, plus I told myself I didn't want to get overloaded with "stuff". I currently have about 50 pieces of rolling stock and 10 engines.
The thing I would like to have a do-over on is the layout. I've already changed layout designs 5-6 times and still haven't finished the scenery. There's only so much you can do in a 12x12 room, but I'd go with 090 or so curves on the mainline and 072 on everything else.
I think if I redid my design I would make the mainline go twice around the room using elevated track around the outer perimeter with industrial sidings on the inside, to make it appear the train is coming from somewhere offline.
If I could make some suggestions it would be:
1) lay down some track and DON'T do any scenery for 1 year, especially ballast. Run your trains and see how you like your track plan.
2) use as large a radius as possible, this will save you money in the end.
3) decide if you want to run trains around a loop, switch some industries, or both. I like doing both, but with my short mainline running loops isn't as exciting as it could be.
4) spend wisely and you'll not have as many regrets.
5) don't be afraid to try new things. Buy an airbrush and try some repainting/lettering. Take a boxcar and modify it for something your RR of choice used.
6) PUT THE MODELING BACK IN MODEL RAILROADING!!!
I've thought about going 2-rail, but the 3rd rail and large flanges doesn't bother me that much, plus the availability of engines and cars is plentiful. I do however look for things that are scale and prototypically correct and use fixed pilots and Kadee couplers.