Your friend is probably talking about one of the Sennheiser MKH mics. Possibly the 416 or the MKH40. These are great mics, but are perhaps overkill for what you are proposing to use them for. Often people use these because they are recording at extremes and need the low noise (such as recording quiet atmos at night) or the resilience these mics have. For sure, they're a mic for life, but I think you really don't need to spend that much to get good results given the application you state.
First off, I think you should be looking for a stereo pair instead of one mic. Stereo recording is just more exciting and interesting, and throws up more options. We've got two ears afterall, and we're used to hearing slightly different sounds in both of them. For experimentation you can either embrace or throw out conventional stereo mic technique, depending on your style. For example you could record the sound of striking a pan in your kitchen, with one mic above the pan and one below. Stretch that out 500x with Paul Stretch and you'll probably get interesting phase fluctuations between left and right.
Probably the most flexible and low cost solution is to get a pair of pencil condenser mics, which cost about £240:
They're good for recording most things, fairly small and dependable. Pencil condensers like this also have the capsule close to the end of the mic, so you can get them very close to sources, which can be useful for this sort of thing. Even if you upgrade later I think you will still have use for these. there should be plenty on the secondhand market too.
Another mic I've used quite a bit is the DPA 4060 (high sensitivity). I bought a stereo pair of these after going to a sound recording workshop with Chris Watson, where he was raving about the sound quality and the flexibility of using miniature mics like these:
Don't be fooled by the size, they can record sound as full and rich as larger mics. The advantage of these is that they can go where other mics cannot, you can attach them to surfaces and get them extremely close. You can hold them one in each hand and move them around sound sources at will, no need for tripods. I've never heard of anyone regretting buying these. They're around £600 a pair. In my experience they do pick up something above 20khz too.
Another option, though I've not used them - Earthworks do a range of mics they label QTC which are designed to go over 20khz. I've heard good things about themfrom other sound designers.
Another quick tip - you might have fun recording with an inexpensive pair of contact mics. Lots of fun to be had there recording unusual sounds around the house.