Your main problem is going to be getting a non-Japanese spec laptop (mainly keyboard layout and Windows version). 99% of laptops will ship with Windows XP Japanese Edition, but if you're not too bothered about re-installing Windows (or another OS) yourself you could do this when you get back. You may get one or two problems tracking down obscure device drivers though.
Alternatively there are shops in Akihabara that specialise in 'overseas models'. These are not necessarily quite what they seem. They're generally made for sale in Japan but to foreign specifications. The choice can be a little limited to a few manufacturers though. Toshiba are probably the best supporter of overseas models in Japan, but you can get IBM, 'Sony Overseas' and Fujitsu, and maybe others. Personally I have a Tosh - they have a strong history in the portable market and make very solid and reliable units. I also got it for about 40% less than the UK price (my home country).
Power requirements won't be a problem since laptops are designed for global travel so you can plug it in pretty much anywhere in the world and it will auto-switch.
You might want to try a few smaller shops in Akihabara. Although people say the Japanese don't normally barter over price, I've found that you can knock people down quite a lot if you push (or alternatively they will throw in a bunch of extra stuff). The bigger chain superstores are unlikely to do this as much though.
One thing that is really important is that you get the shop, whether its a specialist shop or mainstream electronics store, to fill in the duty free tax form which they will staple into your passport (you will need this with you at the time of purchase). This allows you to avoid paying sales tax, which at 5% can run into tens of thousands of yen savings on a high-end machine. This is supposed to only be for visitors to Japan, but actually I did this myself even though I am resident here. I just tore the form out of my passport after leaving the shop. No-one's ever caught up with me, but don't tell the authorities that!
One place that advertizes frequently as supporting foreign buyers is "Users Side" (conveniently in Akihabara). I can't answer for their service because I've never shopped there, but their website (which sucks a little) is:
http://www.users-side.co.jp/index_eng.php
Hope this helps.