I'm just chiming in now, as my heart had stopped, and I had to use a defibrillator to jump-start it back up again ... Wow ... you have quite the treasure.
All great comments from our forum guys above. I'll add a few more points ...
1. The tank/loco is Lionel #203, is suspect Type 1 from 1917. Type 1 was olive (as yours) and had "Lionel Manufacturing Company" on the bottom tag (as yours). Type 2 were battleship grey, and Type 3 (1918 through 1921) were grey with a "Lionel Corporation" tag. Your #203 Type 1 is very rare, indeed, with no price reference in the Lionel guides.
2. The #203 was teamed up with a pair (2) of the #900 Ammunition Boxcars to constitute freight set #214, which I believe is what you have, circa 1917 or 1918. The ammo boxcars were grey (rather than olive), I think confirmed by your first photo.
3. As side notes ...
a. The #200 ammo cars appear to be similar in form to the #800 boxcars, with the #900 being much more rare.
b. The #203 loco/tank was also sold as set #215, which included a pair of baggage cars (#702), rather than the ammo cars.
4. Re: paint and finish. Absolutely do NOT attempt to strip off any paint, rust, etc. Leave as original an untouched as possible.
5. Reck has a good point about the risk of moisture in plastic bags. I've wrapped a couple of old trains I have loosely in cotton micro-fiber towels, then placed them in a rubbermaid-type bin, along with some silica gel.
6. As you begin to contact reputable Lionel dealers, etc., make sure that they are TCA certified (Train Collector's Association), or can give you good reason as to why they are not.
7. You might want to contact the National Toy Train Museum to ask their advice on what to do next -- whether you consider preservation or eventual sale. www.traincollectors.org
Thanks for sharing this fabulous set, and please keep us posted on your thoughts.