The problem with a "minor" oil leak on a 986 is that it is potentially a sign of bigger problems. If you didn't have a PPI done by a Porsche shop before buying, you should get to one, and have them put the car on a lift, and inspect it thoroughly.
Google 986 RMS and IMS or search this forum, you'll find a lot of info that will keep you up at night (you're welcome :P).
To access the engine from the top, here's a write up, and a video:
Pelican Technical Article: Boxster Air Filter / Pollen Filter Replacement - 986 / 987
(this site is an excellent resource for many boxster maintenance topics)
Porsche Boxster S - 986 Remove Engine Cover - Replace Air Filter porscherepair video - YouTube
But, you may need to put it on a lift or at least jack stands to see where you're leaking from...
To reset the break wear light, you probably need to replace the wear sensors in the bakes themselves, which is about a difficult as replacing the brake pads -- fairly straightforward if you are remotely handy. See the Pelican site for the brake pad replacement steps & you'll find the wear sensors mentioned in there.
There are a few after market tops with glass, search this forum or ebay, you'll find them; Price varies but I think you're looking at around $600 - $1000