1. No, there shouldn't be any problems doing that. The mandrake/mandriva normally asks you where you want to put the install. I, for example, have my hard drive set up with a /boot, /swap, / and /home partitions. I would just tell it that I wanted the install put in the / partition. Of course, you also have to make sure that all partitions (and hard drives for that matter) are listed in the /etc/fstab before you finish, otherwise you won't be able to mount them (mandriva normally "automounts" the drives/partitions - it uses the fstab to know what too mount).
In my example, the install would just overwrite the older version. Provided that during install, I installed all the same software packages, then any icons/links in my /home will "just work" (they don't usually change the commands that are used to start the apps). The /home is just left completely alone during install i.e. not formatted/changed in any way - so then any customisation/data/address books and other personal settings remain the same.
2. I can't really say. I've never used SATA hard drives, though I understand that they can occassionally have some issues during install. Though it's usual to put the bootloader onto the first section of the first hard drive. That way, it should offer you the choice of OS's too boot. It overwrites the windows MBR with the Linux MBR. This isn't a problem, because if there is a problem whereby you are having problems booting into windows, then you just boot the system with the Mandriva disc, and instead of telling it to install, you follow the instructions for "other options". Then just type in "rescue" (no quotes), and end up with a list of options that includes "re-install the windows bootloader". Then you can get back into windows. Then you just have to find out about correcting the problem of booting the linux. There should be no need to play around with the hard drives as such.
3. You should have little problem getting Mandriva onto the T43 as a dual boot in the same way as you would with a desktop machine - Thinkpads are supposed to be pretty good. Personally, I'd just install linux on it, then if I had the need to use some MS apps (office etc etc), I'd just get "Crossover Office" from Codeweavers and install the MS apps with that. Then you can use the office apps, but under linux. Or better still, install Open Office 2.0 and then just run the windows documents there. If you're just concerned about keeping installed apps/data, then just shrink the partitions of the windows install to something like 20 or 30 gigs. Put the linux on the rest of it. If you needed to have extra space for data (under either OS), if the external drive is formatted as FAT32, then both windows and linux can write too it without any problems - you just have to make sure about the linux install seeing it as an external drive.
4. When I first started with Linux, I used a download version of Mandrake 8.2 - I did have to learn to do some stuff manually. Though I found that I enjoyed using Mandrake (now mandriva, but I'm sure you know that). So when the newer version came out, I got a "boxed set" version of 9.0 directly from them. doing it that way, negates the need to tie up your connection, and you only need to get the updates and newer versions of available packages via the connection. If you look here
you'll find that the "boxed set" is about 80 euros (you'll get it priced in $'s if you're not in europe). Though if that seems a little dear, what I used to do, was to dig around their site and you can usually find the "workstation DVD" version. It's cheaper, as you just get the disc in a case, no manuals etc etc. Then you can always check out any problems either here at LQ or you can look at mandrivausers
. That was the cheapest way IMO - the "workstation DVD" disc only one is here