Has anybody used Pentax K--EOS lense adapter?

I have an old Sigma 600 mm mirror lense that has a Pentax K mount. I see I can get an adapter ring to fit this lense on my EOS (300D) body. I figured the adapter ring is relatively cheap so why not get it and play with the lense, but before I pop for the adapter I got a few Qs. Has anybody here used such an adapter? If so, can you tell me any tricks to selecting camera mode? If I use aperature mode with this fixed f/8 lense, can I get metering/auto shutter speed to work with the lense/adapter on my 300D?

The camera won't know what aperture the lens is and cannot control it. (obviously nothing could control a fixed aperture lens)
So just put in Av mode and camera will select a shutter speed.
If using a variable aperture lens , just manually adjust aperture.
It's easy to simulate how it will function - just remove your lens and see how the camera behaves.

The K to EOS adapters are hard to find, or they were when I bought mine, and also quite pricey compared to the likes of an M42 adaptor. I use mine with 50mm 1.4 and 1.7 Pentax lenses and they work great in AV. Lovely lenses.

I got my K to EOS adapter ($35), it fits great, but now I have a question. When I put my 600mm f/8 lense on the body, the meter reads aperature "00" in aperature, shutter and manual modes. I can't dial in an aperature. This is a fixed f/8 aperature lense, so I want to tell my camera f/8 so I can use TTL metering to dial in shutter speed. How can I tell my 300D camera f/8 aperature? Or am I limited to hand light metering?