I just saw this thread and felt I would chime in. I'm by no means a professional sculptor but started three years ago and moved from Red Graf's situation to someone who can produce decent gaming minis (Vermis might still remember my early attempts and I'm grateful for all his help when I started). Below is my last sculpt which will be part of a diorama for the LAF Build something contest.

As the thread is quite long I might have missed what others have already said but I will cover my experience with 1. putties and clays. 2. tools 3. some sculpting tips.
1) I'm a big fan of green stuff (GS) and I don't like sculpting with clays. The whole process and feel is radically different between clays and putties and I won't say much about my limited and largely disastrous experience with clay. Anyway, clay is not an option for minis that are are meant for gaming rather than casting.
For a while I was mixing GS with Fimo or Fimo Soft but I stopped doing so and now use pure green stuff in roughly 60% yellow/40% blue proportions but I don't really pay much attention to the exact ratio. I find the short curing time a blessing. It is about two hours at room temperature and if I fail to sculpt something in two hours, I won't succeed in twenty. Actually I've ruined quite a few good sculpts by not stopping to fiddle with details in time. Also, I like the fact that the consistency changes over time. Initially it is soft, which makes it easy to get the major details. As it hardens it becomes easier to smooth.
The curing time is also quite flexible. I often accelerate it by putting my minis in the oven at 50°C, or if I need a break or get interrupted I put the sculpt in the freezer, which extends the curing time to about twelve hours. GS is the only sculpting medium I know which allows creating minis sturdy enough to resist the attention of cats and small children. Incidentally I disagree GS can't be filed; it is just a bit tedious and requires using sandpaper with an extremely fine grain. However, I stopped doing it as I've realised that cutting and sculpting anew the problematic bit is just much faster and more satisfying. Finally, don't forget to store your GS in the freezer, as it hardens at room temperature.
As others mentioned, brown stuff has been discontinued, but there is grey stuff which has somewhat similar properties. It's fairly easy to find as it's marketed by Gale Force 9 towards beginners. I recently bought some out of curiosity but haven't used it much so far. I feel it should work fairly well when hard edges are needed. However, I wouldn't recommend it as the material of choice for sculpting female posteriors.
2. You need very little equipment to sculpt. I've got plenty of tools at home but most just tend to get in the way and 99% of times I use my standard sculpting tool and a needle mounted on an old paintbrush or a mechanical pencil. I'm away from home during the week as I have a very long commute to work and have a traveling sculpting toolbox, whose whole content you can see on the photo below.

The two sculpting tools (on the left) are the new GW one, which I like but is clearly overpriced and the old GW one modified probably in a similar way to Vermis', which I've essentially stopped using . A big improvement in my sculpting coincided with my acquisition of magnifying glasses; I can now see what I'm doing, which is quite helpful. The ones in the picture are very cheap ones but include a light. At some point I might improve on these by buying professional binocular magnifiers like the ones used by dentists.
3. I'll be very brief on the sculpting tips but if there were some general interest I would be happy to post a detailed progress log on how I sculpt a mini. Just a few things, first you always need an armature. Even for a conversion like yours you will need a wire armature to connect the legs to the body. You can achieve this by drilling small holes in the body and legs, or with plastic you can heat a needle and just make the tiny holes. I always cover the metal with superglue (and wait for it to dry !!). This will give a good grip to the first layer of GS. Always sculpt one layer after another, e.g. a) first layer to cover the bare metal; b) muscles, c) fat and skin, d) cloths. Sometimes you can cut corners and go straight to the next level but it generally pays off to get the lower levels (in particular muscles) right even if they get covered by clothing.
I would avoid water as a lubricant. Saliva is actually pretty good but I'm not sure I should really recommend it; vaseline also works a treat. If using vaseline, don't forget to wash your miniature (with soap and warm water) in between sculpting successive layers. Finally always make sure you have a clear mental image of what you want to sculpt and look at many images of your subject. The internet is great for that and I guess you should be able to find some pictures of scantily clad female bottoms there.