Ah… And do not waste money on SSDs. You won’t have any advantage. Wireless is slow, USB is a bit faster but not comparable to SATA interface (regardless if SATA I, II or III - okay, if you have connection via USB 3, that SATA I and II will be slower). WIFI brings up to 1 GBit / s under ideal circumstances. USB 3 will bring up to 5 GBit / s - but has some latency and overhead. SATA III will bring 6 GBit/s. It doesn’t matter if you put a SSD into the case or not - it’s just waste of money and reliability. The TimeCapsule is not a NAS - it’s intended for use as backup device. Use a enterprise grade hard drive for backup /archiving. It may be a bit slow compared to NAS drives or SSDs, but for TimeCapsule it is fast enough and - more important - extremely reliable.
Interesting but unnecessary. Just buy a standard USB 3 2.5” drive and connect it via USB to the capsule. Won’t look really fancy - but devices like a TimeCapsule are normally placed out of direct sight. The advantage is, that you can easily replace the external drive and extend it with a better model from time to time without risk of damage and within no time. External 2.5” drives are available with drives up to 4 TB and do not need external power source. So you would have the internal memory + external memory. It’s that simple ;-)