As far as I know, everything from the two developers you mentioned works in FSX:Steam.
If you install FSX:SE stand-alone, on a system that doesn't have FSX installed (so if you already installed FSX, you must Uninstall it first), the FSX:SE registry keys will be identical to the regular FSX ones, and this usually fix all the possible issues that some older installers might have, not recognizing FSX:SE. And this is usually an issue with old installers of products that have been abandoned by the developers long ago.
With FSX:SE, I gave you the absolute cheapest solution right now, which would give you also an online "backup" of FSX.
About P3D, it's clearly the most sensible choice and it's a far better sim, and one which has a future, while FSX is rapidly declining. However, you'll have to check with all your addon vendors, if they support P3D and how you can update (there might be some upgrade fees, or not, depending on the product)
Don't even bother with older P3D versions, after P3D4 came out, which is a 64 bit sim, it doesn't make any sense to consider an old version.