Placing TMCC in a 1983 engine

I will try to answer.

a) the sp engine ...

I am not familiar with that engine, including the part about it being DC. If it really is, then it might be a cheaper conversion.

Regardless of whether its AC or DC, most likely there is not enough room inside the engine and you have to put the electronics in the tender. And since the electronics are in the tender, you need a way to get the signals into the engine, and then a connector so you can take them apart.

All that is a lot of work.

The place to start would be the Electric RR, and come to think of it, they might be the only TMCC conversion house left; know that the original owner is now an employed at Lionel, Jon Z.

Personally, I would sell it and use the money towards some used SP Legacy engine. That is of course an opinion, I am not in your shoes.

b) recommendation.

... I have none.

c) running four long passenger trains

Assuming the trains have plain old light bulbs, and assuming you are mostly TMCC, I would go with 180W bricks, from two up to four of them, depending how deep your pockets are. Then I would put something between the brick and the track. Might be a ZW-L, might be a TPC unit, but my new favorite is the Lionel Legacy PowerMaster.

There is also some silver 'doghouse' thing that can go between the transformer and track, but I can't remember its name, and it clips on only to tubular track.

The above are all opinions.

I'd be surprised if that was a DC motor, most in that vintage were AC motors.  It's possible to upgrade it to TMCC with either motor.  As Mike says, the electronics will doubtless go in the tender with a tether.

As far as the power requirements, exactly how "long" are the passenger trains?  How many lighted cars, and how many bulbs in the cars?  For the most part, if you're running command, two 180W bricks would run any reasonable assortment of four passenger trains.

Your engine is like the N&W J.  Good chance it can get a Command Unit like the LCRU in the engine and Sounds in tender.  Even can get fan driven Smoke unit.  I have converted the N&W J before.

E-mail in my profile.  G

I have had experience with both "Gunrunnerjohn" and "GGG", either of these guys will do a really great jobs on either an upgrade OR repair of your other engines.  I personally would have them do my stuff than any service repair facility at a train store.