Chuck, welcome to the forum,
A bit more info please…how big of a layout are you thinking?
I really dont know why every knocks brass track, stainless to me is just more money. Makes sense if you are going to hand lay track and scratchbuilt all your switches, and operate track power, but for a eventual battery oped layout? Its just money down the rabbit hole.
Keep it simple to start, a loop with an LGB powerpack and then switch to battery when you can afford to convert one engine, then the other. Once you are completely battery oped, then add switches, reverse loops, passing sidings, etc. with Battery power you can ignore all the hellish wiring cunundrums that come up trying to do track power to a large outdoor layout. Just lay track and forget it.
Flex track is really best for large spread out layouts, the rails are in 5 foot lenths and needs a $100 rail bender to make curves.
A more compact layout is better served using sectional track, its cheaper in the long run and easier to work with.
How wide are the curves going to be?
With the locos described the standard R1 4 foot diameter curves work well, but I would recommend if your outside and have the room, use 6 foot MINIMUM diameter, and 8 foot diameter is more preferable. Almost every new item coming out (DASH-9, PA-1, Pacific, Connie, Heisler, etc) is being designed the 8’ diameter curve, and most older stuff (GP-9, FA-1, F-3, Annie, etc) will work with the 6’ diameter track. So if you have the room, Go Big with the curves you wont regret it later.
If on the otherhand you dont have the room for the big curves, dont be put off by the 4’ diameter stuff. Thats what I use, along with LGB 1200 swithces. I also use the LGB Porters, Bachmann Porters, MDC Hustlers, and they are all great on the small radius stuff, just plan on using 20’ or smaller cars and shorty Sierra type passenger cars.