There is no way to estimate this because you don't own it and have no idea what is underneath the existing siding. The only thing you can do is a worst case estimation. Call a couple of contractors and ask for bids to remove all of the siding and completely re-side the house. Tell them you will go with the best bid if you buy the house. At least you will then have a maximum idea of cost.

Agree with Kudzu, assume the worst and hope for the best once you remove the siding.

We also have the original cedar under our vinyl (we think).

To give you a head start, we got a single bid for the same thing. Plan to get a couple more. It would probably be a decent amount of work filling nail holes from vinyl, etc, and who knows what condition it is in. I assumed the worst and got a bid to replace with 3" Hardie lap on bottom 2/3 with metal corners and shingles on top. Was close to $20k (not including painting), and my home is 1400 sq ft in Seattle.

We paid 12k to reside a 2k sf ranch in Wisconsin. We had ceadar board and batten torn off, the house wrapped in Tyvek, and resided with LpSmatside board and batten panels that we're pre-finished with out color before installation