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.