Hi Greg
Glad your son has taken up the hobby. It's how my dad started me off with my first set in 1958.
 By "Controller" I assume you mean Transformer? For right now the cheapest and easiest way is, If he is going to stay with the hobby, then pick up another transformer that has  multiple output(s). Lionel makes many.
What usually is done is, as the layout becomes larger or permanent, you either upgrade to a multiple source transformer like the ZW (Which has 4 outputs) or one of the others that has multiple outputs.  Most folks will put their accessories on a separate source, Another output is used for constant voltage Power for  your switches and yet another 1 or 2 outputs for train control depending on how many isolated track systems you have.  I have more than one transformer running and I won't get into that right now. It takes a little bit to correctly set that up. Lionel has videos on all of this you can watch.
Claude

Hi Gary
 I see this post is over a month old now, but I will try to help you if I can.  The 6-8111 Came with a "Select-able" Reverse lever.  I have an 8111 myself. It was a bit of a clunker when I picked it up. What I did was put a 2 position E-unit in it. I don't know what number it was, I just had one laying around from an old post war alco engine and soldered it in.  Might have been something like a #101-1. 2 position, short lever.  I just can't say exactly.  Before you start you might want to look for an exploded drawing of the 8111 and get all the part numbers you need. I think I had one around somewhere but not sure where it is anymore. As far as the nylon Gears, they are very prone to wear when put under heavy load. There are replacement gears for it.   It takes a bit of work to do, but if you like that part of it, then it's ok. You may need a wheel puller, not sure. You might find a whole motor truck for it somewhere too like on e-bay or Trainz.com. and change the whole thing out. The cost may be a bit much depending. I would run it for now with light loads and not pull heavy grades under load. Lubricate it according to the instructions. You might be able to find an entire working chassis and motor for it too. Swap the shell.  Brushes for the motor and all that can be found at Trainz.com or Ebay.   

It's a 2343 Postwar Santa Fe F3.  One screw in the bottom front, 2 screws in the bottom  back and the shell comes off. If your brother installed a replacement horn You will have look at the installation, trace the connections and see how he did it. Look for corrosion, broken wires, battery damage ect....
 If, it's the original 2343 horn setup and the horn itself If its still there, it will be above the rear motor on a raised bracket..  There is also a relay that it connects to as well as additional wiring. .  Sometimes the copper contact on the relay corrodes and stops the horn, Sometimes a dead  battery  left in the engine for years causes damage to the Battery  holder compartment or it is just  plain corroded. The horn itself may be oxidized a bit and not vibrating. Don't take it apart!  Read up on how to do it.......The little tabs holding the cover on will likely break off if you don't and you will never get it back together so that it works without replacing the cover. See which horn setup is in the engine

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.