Here are my answers to your questions... Track - USA track is the best - https://standardgaugetrack.com/. Radius depends on the equipment you want to run - #10 engine and 500 series cars can run on 42" diameter track (42" is "original" SG track); larger trains are more reliable on at least 72" diameter, and USA track has much wider curves available. Cars and locos - MTH made a wide variety of items, pretty much whatever you want, most locos came in traditional and contemporary (PS 2 or 3) versions. The "Tinplate Traditions" items are generally a little less expensive - they're identical to the Lionel Corporation versions, but lack the "Lionel" markings since they were sold prior to the licensing agreement.
However, as a lover of Standard Gauge trains, I suggest either going with prewar pieces - simple to run and repair - or going with a different gauge. Modern SG trains need frequent attention for lubrication, and keeping the gears greased on locomotives can take a fair amount of disassembly. In addition, parts for modern trains can be difficult to find.
For an operating display that needs minimal attention, I'd suggest G - it runs reliably with less attention and maintenance. A second option would be O gauge using Williams locomotives - simple and bulletproof.
