5G and 5.5G both use wolfson DAC afaik. Best thing for you to do would be to get a broken 60GB or 80GB model with a broken hard drive. They have the larger memory to allow buffering of additional data and whatnot. The hard drive is really the most expensive part of fixing up an iPod. The logic board can be had for around $30 - $50 depending on whether you find a good deal.
I'd recommend you purchase broken iPods from a dealer who doesn't sell their own *good* iPods. I've not had good luck with these for the obvious reason if it was good enough to get working with ease, they probably would have done it. I've had better luck with people who have low feedback numbers who are selling their own iPods because they broke, or sellers who sell random stuff and came across some bad iPods.
Try to get units that power up and show a folder or sad iPod icon. You don't want to mess with units that don't power up because it could be water damage. Although, admittedly my 2 or 3 best finds were units that didn't power up, because some connector or other was off the board. I just had to resolder the part or just reconnect a cable to get a full working iPod, but that's pretty rare.
When you're looking for 5.5G vs 5G, keep in mind the only advantage is the search function and a brighter LCD, no other real differences afaik.
Something else to keep in mind as you're planning for this... with a broken iPod you might not know which part is broken and end up spending money on parts that end up not resolving your problem. But that's the risk you take with this route.
For the cases, you have to be careful because some of these replacement cases are just down right atrocious. I found one seller that I really like (theraininc) and have been ordering all my cases from him. I've gotten one bad batch and he replaced them with very little hassle with good ones. You can find most other parts on there as well (no, I'm in no way affiliated with the seller, I've just had bad experiences with random sellers that I don't want to see others repeat).