I just replaced the driver front seat bottom on my W123 last weekend. Took about 90 minutes.
The front seat spring box, and pad are readily available. Cost for both for a single seat should be about $150. The bottom cushion and spring box are VERY easy to replace. The MB tex bottom cover pops off and on quite easily. You won't need an upholstry shop for that. The seat backs... Dunno... That looks scarey.