Finally fixed the phantom horn syndrome on the Boxster. Thought I fixed it last week, but hadnt used enough Tiger seal. 
Also fixed the window drop problem. The window is supposed to drop slightly when the door handle is pulled, to allow to to drop out of the recess on the hood, and then lift to original position when the door is closed again.
A dose of contact cleaner around the microswitch and connectors seems to have cured it.
Omega is getting neglected somewhat with another maintenance hungry car on the fleet, but I dont use it that often, so hopefully it isnt deteriorating too much.
Was planning to do quite a few little jobs on it this summer, but that will probably run to next summer now.
For what it cost you've had very few expensive problems with your Boxster.
Yep , its been reasonably cheap to run for what it is. Easily comparable to running an Omega.
My one big worry was the cost of tyres when they wear out.
A couple of weeks ago I bought a set of normal 17" Boxster wheels with a pair of brand new Toyo Proxes tyres on the rear for just over £100
Ive just sold the 18" Sport Design wheels I took off (with pretty worn Jinyu,s on) for nearly £500.

The indicator switch gave trouble recently - wouldnt stay on when indicating left. Usually around £150 second hand for a replacement.
Dismantled it, found a little piece of broken plastic and glued it back into place with a hot glue gun. Works perfectly now.

With quite a lot of parts being silly money, it concentrates the mind not to play parts darts.