Personally, I'd wait and buy the TIU/WIU or TIU/remote from an authorized dealer if you go that route. Then you receive a substantial warranty, which can be reassuring and convenient when you are investing this kind of money.
I agree with Stan. If the goal is to operate two trains simultaneously, the TIU/WIU may be overkill as the Explorer should meet that need. A couple of switches can be operated by lots of less expensive modes than a TIU/WIU/AIU.
Then again, if the goal is to have everything on one remote control device, the cost of the TIU/WIU/AIU may be worth it to you. Unless you are very knowledgeable and handy, the secondary market for such devices can lead to disappointment and frustration.