I did the SSD upgrade on my iHP-120 using a Kingspec 128gb SSD, and replaced the battery with the Cameron Sino 2200mAh whilst I had the case open. It was mostly straightforward.
I'd already previously flashed Rockbox to the player (V6) and it was working fine. The case opening and battery change instructions at http://www.pdainternalbattery.com/iriverinstall.html have the info about opening the case and stripping down the player to get at the battery. I copied the .rockbox directory off the player before I started.
I did the battery first. The battery is attached to the case using strips of double sided tape. I used the handle of the tweezers to put underneath the battery and carefully ease it off the case. I then put a small new piece of double sided tape on the case to hold the new battery.
Getting at the battery was tricky, but the instructions from the link above are good - but be careful and patient, the screws are tiny!Getting the battery connector out was difficult - there's *just* enough room to do it. I put a pair of tweezers around the connector from the side, then eased it back and turned it upward as I did, and it came out. It wasn't easy, the connector was a tight fit, and felt like something would break before it came out.
Putting the new connector in was slightly less awkward, but be careful you get it lined up properly - it looks like it could be easy to bend two tiny contacts in the battery plug socket that plug into the new battery connector.
After putting the motherboard back, I put the old HDD back, and closed the case with a screw either end, then turned it on to make sure the battery was OK and the player worked. I then gave it a charge for about an hour to be on the safe side. Then I opened the case again, stuck the battery to the new double sided tape to hold it in position, took out the HDD and replaced it with the Kingspec SSD. When I put the case back together, the rubber anti-vibration cover that was over the old HDD made the fit too tight with the new battery for the case to close easily, so I left it out.
I then connected the iRiver to my PC, erased the factory NTFS partition, and formatted the drive to a single FAT32 partition using Swissknife. I then copied across the .rockbox directory, unmounted the player from the PC and booted it up.
It worked first time.
Overall impression so far is that the access time is much quicker than the HDD. I've been using the optical line out to my DAC, and battery life seems to be improved, so it's given the player a new life.
The HDD-SSD swap is easy, the battery replacement more tricky. You need a T-5 torx driver, and I'd recommend a pair of reasonably fine tweezers to help get the battery connector out.
These are just my experiences of course - you do the mod at your own risk.