I'm no expert but I if i can make a few suggestions to get you started with the process. You might want to pick up a copy of Greenbergs pocket price guide to Lionel trains 2025 edition.  You can get it in a lot of places. It will give you an Idea of about how much certain items are worth.
 I think that selling them to the right people is the key. Things like condition and rarity are always the big factor. Some places just buy collections bulk and are looking to get them as cheap as possible for resale. Collectors might pay a bit more per item but you will have to connect with them individually and it will take more time to sell them all.  Search the internet about how to go about selling Lionel collections and see recommendations on what to do by people who have done it.  Hope it turns out well for you.

Hi Dockside
Have you found an answer yet? Typically the tender numbers are ink stamped in silver on the bottom of the chassis. I do recall 2 postwar tenders with 6 wheel trucks. After searching I cam up with the #2426W which went with the #726 and #723 engines. and the #2671W Pennsylvania Tender possibly paired up with the #671 and #681 engines.  Not sure of any more or any newer versions

Hi Ron
Space constraints are the biggest problem to any model railroad. We'd all love a bigger layout than what we have! LOL 
 Buying a set is probably the cheapest way (as you said ) to get everything including the transformer which can be a big cost individually. If you do buy a set and all of the rolling stock and engine can run on a 31 inch minimum radius, you can still use the 10 inch fastrack straights that come with the set and buy 31 inch radius curves to use with it for about $4 a piece.  Maybe cheaper on eBay. Sell the 36 radius curves off or save them for when you have more room.