1. 4200 is the slowest, and nowadays, usually at least 5400 is standard. I'm not sure why HP is still putting 4200RPM drives in anything but their cheapest or smallest notebooks. If you're going to use it as a true desktop replacement, the upgrade to 7200RPM is a very good one.
2. They're not exactly the best in the industry. They're definately not the worst, but the company they own is one of the worst. That's a different topic, however. Anyway, HP is not as good as some companies with models in the same price range. I can't tell how much that particular system you were looking at is, as it led me to an Italian page, and I can't find the same system on the U.S. site, but from what I've seen, you can get better systems for cheaper.
3. That's all personal opinion. I like Intel better, but if I found a laptop that was perfect for me, and it had AMD, I would not care one bit.
4. I don't necessarily think it's underpowered, but the video card is not that great, and the hard drive is slow.
5. Desktop processors and notebooks don't go well together. Desktop processors run hot, and the battery life sucks. A Pentium M may seem slow on paper, but actually, a 2.0GHz P-M compares to a 3.4GHz P4. With plenty of RAM, I doubt you would notice a difference in multitasking. The Turion is a good processor, too. Don't worry about P-M vs. Turion. Find a laptop that has everything else you need, and if it has P-M, then so be it, and if it has Turion, then so be it. Just avoid desktop processors, which includes the AMD desktop processors.