A few things folks haven't mentioned yet.
As long as you stay with MTH, LC, and LC+, you'll be fine. This basically means that you'll always have full voltage on the tracks, just like your current LC/+ setup. The DCS remote will not be able to control the LC engines; the power provided thru the DCS system (TIU) will be fine; you'll just need to use the LC remote. The LC+ will also need to be controlled by its remote, unless you want to operate in conventional mode, and that's a topic for when you gain a bit more knowledge (not hard; just don't want to confuse at this point).
You should be able to run both DCS and LC/+ engines at the same time. Will take 2 remotes (DCS + LC) and some skill in avoiding collisions, but can be done.
You didn't say what kind of power supply you currently have. If it's just a transformer with no controls that you plug into a section of Fastrack, you'll need a different source. If you have a traditional-type transformer like a CW80, or a PowerHouse brick, or an older transformer, any should work to start with for one engine. At your level of understanding, I'd advise against an older transformer because they need some protective devices installed which is beyond the scope of this thread. Easy to do, again, don't want to burden. But, if you have something like an old ZW sitting around, it'd be worth it for you to search on how to add spike suppression and fast-blow circuit protection. VERY easy to do. Don't have to open-up the case, either.
All comments have been spot-on and valid. Get Barry's book before you buy anything. Just about all our sponsors at the top of the page offer the full DCS system for around $309.