Most of the caps on most of the IIfx's are tantalum from the factory. So there's very little, to nothing, to replace.

IIRC, the IIfx needs two good PRAM batteries to boot up. I could be mistaken.

If all eight slots are full, then you could have 8 or 20 MB of RAM, but 16MB is not one of the possible configurations with 8 SIMMs installed.

As with other Macs of this age, any SCSI hard drive with good termination should work. One of the adapted, new(ish) SCA SCSI drives with appropriate adapter could also be used.