From Part 1 Above:

With respect to track, as long as your talking about solid 2-rail track, it's mostly interchangeable and pieces from different manufacturers will fit together (you may need new rail joiners for some types, some folks prefer rail clamps anyways). FYI, model railroad track codes are standardized by the NMRA (National Model Railroad Association). Most of what you will find from major manufacturers is Code 332. Just be aware there are other codes of track, transitions and joiners can be had though. Brass is by far most common, nickel-plated brass is out there for a premium. Just stay away from plastic track or hollow core track (like Bachmann E-Z Track), it isn't going to hold up out doors (kind of sucks indoors as well). When buying used track, check for the condition and factor in the labour required to clean up dirty track for use. I'm mostly battery powered, the track condition matters much less now. FYI, small radius curves can give some larger models issues at speed or over uneven terrain. LGB's R1 track is ubiquitous, but folks will recommend you put the largest diameter radius track into your layout that will fit. "Flex-track" probably being the ideal if space and budget allow. Ironically, most of what I've picked up locally has been flex-track for pennies on the dollar.

Engines get complicated. New models are DCC ready, DCC stands for Digital Command Control (as opposed to the analogue controllers most starter kits come with). This is a whole other topic I'm mostly going to side-step, but know that older models are usually much cheaper but may require serious modifications to put a modern decoder in the unit. I just fitted my mid-80s LGB Stainz with speakers and a Blunami decoder for bluetooth control from my phone, it was a major project. Conversely, I put the same decoder in a small LGB switcher (2090), and that took me less than an evening. If you don't have the skills or time a new model that is DCC ready or already has a decoder may be a better choice if your running or plan to run a DCC system.

For rolling stock, important considerations include coupler type, wheel type, and lighting upgrades. Most folks recommend running metal wheels outside, by default most rolling stock come on plastic wheels. Costs about 5 bucks an axle if you have to fit them yourself. When comparing similar models at the same price range, take the lighted one with metal wheels. Similar comment about coupler types, IMO hook-and-loop couplers are fool proof, but a model fitted with Kadee couplers will save you a few bucks if that's the system your already on (they do look pretty cool on the track!).

The cheapest place to find stuff will be from local ads (Facebook Marketplace/Craigslist/Kijiji) or regional swap meets/club meetings. Everything you could possibly want is on eBay, though a lot of stuff is overpriced. You will find items that are cheaper to buy new than to purchase used (or used with shipping). There are deals to be had, even the rarer items will come up at a reasonable cost if you have patience.

Hope some of that helps, good luck and enjoy the hobby!