[dpaul]

It is almost certain that an aftermarket tune has been installed - otherwise you'd have a MIL from cat removal. In the absence of the tuning cable and/or tune files, it takes significant technical knowledge and some specialized hardware to get any information regarding the tune which wouldn't be worth the effort. To address your concern, you would have a problem if you took the car to the dealer and they updated your DME, which they sometimes do. The stock tune would not provide correct fueling and ignition for the installed mods but much worse, you'd get an MIL because the cats are gone and you would not pass future inspections. So just don't take it to a dealer or if you do, insist that no modules be updated.

Like @pbonsalb states, your worst case scenario related to tune/modifications is that you have to remove the headers and catless exhaust components, replace them with stock and flash a stock tune to the DME.

You could probably find used stock exhaust component for (I'll make a somewhat wild guess) about $1K. Replacing the headers is certainly a days labor so add another $800-$1200 or DIY. Probably don't actually need to replace the headers which likely would work with a stock cats/midpipe/crossover. A lot less labor to just replace the post header exhaust. You can do it on jack stands although that is not my idea of fun.

Flashing the stock tune involves free factory software (WinKFP) that runs on a PC (or a VM on a Mac), a $45 cable (you MUST use a Bimmergeeks.net cable) and a 30A power supply (or jumper cables and another car)

Since you have no information about the car's provenance, my biggest concern would be its maintenance history and how previous owner(s) might have used it.