Cost of common fixes:
 

Looking for a Boxster, 90's or early 2000's. Maybe an S.

Is there a dollar figure to do:

IMS
RMS
Clutch
Water pump
SERPENTINE (oops -edit) belt

if all done at once.

I'd like to mentally factor that into my negotiating.

Just looking for a rough ballpark.

Thanks in advance.

These cars do not have timing belts.

you're looking 3k for IMS, RMS and clutch kit with a 3rd party mechanic to do the work. Water pump and serpentine belt are extra. You could actually do that but purging coolant line air is a specific procedure or use a vacuum pump.
You want to ask about chain ramps. They start breaking apart and chains rattle on start up. that is also extra. You want to hear the car on a cold start...first start of the day. If nothing was done, 4k. water pumps are rx'd at 60k now to avoid fin breakage and getting stuck in the head somewhere.

Thank you Tony... 3-4k then. Assume that includes parts.

Have not had a water-cooled Pcar before.

I,m doing this right now parts only all from our host IMS LN ceramic $800 plus oil and filter You can get a new standard bearing similar to original from Pelican for $165
tool kit for the IMS $350 resell on ebay or can be rented from Vertex $75
RMS $65
clutch kit about $325 add $400 if new flywheel needed
Surpentine belt $25-30.
My water pump and clutch were done around 60k
suggest new AOS while trans out $100 but 1/2 hr labor not 3 hrs with trans in.
around here (Tampa) I,ve been quoted $1300) plus parts for above.
Depending on miles motor and trans mounts while trans is out. mine still look good at 67000 but don,t know their history.
I,m a retired tech (but not much water cooled Porsche experience) and am doing it myself

Even with the recent clutch, anytime you're taking it apart again I'd suggest you put in a new TO bearing. It's just a plastic sleeve and it's not expensive.

Looking now at an '02 S.

Has a recent clutch/fly replace (and T.O. bearing) about 2k miles ago.

Their mech thought it had the IMS replaced, but not sure, also there is a faded sticker on the inside of the rear trunk next to the VIN/info sticker (can't read) that he thought was from that. It has a signature on it.

It is not the LN sticker I've seen before.

Could this be for the IMS at some time?

Car has over 110k miles. Car originally from Dallas, now in Colorado, if that matters.

If I do decide to do the IMS anyway, should I do the clutch again?

I have a similar signed sticker in the trunk. Mine is legible and is a dealer update for an emission recall. The emission /vac hose diagram is near it .
I would assume the bearing is original unless you have better proof.
My records do show an updated IMS cover was installed by the dealer under warranty in 2005. The early covers and rear crank seals often leaked and Porsche brought out a better seal. They still didn,t replace the bearing in those days. From what I have read Porsche has never admitted a problem and there is no dealer installed replacement IMSB. Some dealers do install aftermarket replacements though.
On a positive note your clutch should be near perfect and doesn,t need to be replaced at 2000 mi. (Unless there has been an unlikely huge rear seal leak which has soaked it with oil it should typically last about 60,000 mi) If you decide to do the IMS bearing. Don,t buy any parts until you drop the trans. You might luck out and find a replaced bearing