Sorry for being late - it depends on how much the cost will be, but from what I've seen some of the hard drives are as much as a new iPod itself. if you were modding something beyond the hard drive or trying to hook up a 2.5/3.5 in hard drive instead, I'd say go for it, but I assume you want to keep it portable.
I agree with manhackman and w2ed, what is the price factor here? Is it a price factor or are you just trying to make your iPod cooler? In that case, expand your mod a bit.
Okay, the cost of the battery will be cheap - you'll probably spend between $20-$25 through http://www.macsales.com. As for the hard Drives, good luck - if you find one, they'll probably cost as much as a new iPod Nano. (I found 80 and 120 GB, but not 100 GB, in spite of Toshiba's site boasting of one.) The price of the hard drive alone is where you'll eat your costs at.
I also noted, while on Toshiba's website, that they have a 240Gb PATA - wonder if it might be compatible with the pre-video iPods. that'd be a mod worth doing, since Apple has yet to break any ipod beyond the 160GB factor.
Agreed with w2ed research that 250GB PATA, if that turned out you might have to make a bigger back panel but the sheer power of that would be incredible. Or perhaps an SSD drive, and create the ultimate durability iPod. But if you're just looking for a cheep upgrade, I don't think this is your rout.
Honestly, I'd wait and just get the classic later, assuming it's still around when you get it. It's not worth the $70-$195 BS Hassle you'll go through to upgrade the other - you won't see it later if you decide to sell it on eBay or Craigslist, and even if you need it now, getting a new classic will serve your needs down the road. Even though we've not had a 100 gig iPod in any line up (that I am aware of), we've had above that for at least 3 generations, so chances are it might be ignored.
If this were a 240 gig upgrade, I'd be more readily convinced of doing it: the most I've seen any iPods, new or past, without a HD replacement is 160, so from an investment standpoint, selling it later may bring back the money - and maybe more.
If you were doing more of a mod than that, I'd tell you to go for it - but even then I'd tell you to go for broke and get the 240, skipping over the 100 and 120 - You never think you'd need more until you have run out of space. 100 gigs is too small and (more importantly) too uncommon to be worth it.