If the digital speed reading is accurate, someone has been messing with the needle placement on the cluster. To their credit, it's really tough to get it put back in accurately since it's resting position isn't intuitive. Adjusting the needle is probably your best first move. Instructions can be found on my web site:
Porsche Boxster Buyer's Tips, Mods, and More
Horn issues: First check the fuse. My bet is the previous owner removed it because the horn would sound during hard braking. If this is the issue, the rubber bushings behind the airbag have failed due to UV from driving with the top down for years. There are DIY hacks for this repair using a piece of an old hose so you don't have to pay $200 for a new bracket (Porsche doesn't sell just the bushings). If the fuse is in and working, the horn itself might be shot.
I'll let someone else address your tire rubbing issue. Hope this info helps!
BTW, there are a bunch of things you need to check on your 98, coolant overflow tank cracks, oil filler tube cracks, air-oil separator, wheel bearings, mass air flow sensor and oxygen sensors, etc. Start reading up on the weak areas of the early Boxsters because you're going to need to know about it sooner or later, even if the previous owner(s) did all these repairs because they come back.