Are there any reasons not to use a Sierra Estate as the donor vehicle?? Don't intend to use the engine/box as will use a bike engine. Do cars with power steering cause problems???

They don't cause problems, however they are more complex, add weight, take power from the engine, add time to build and are more expensive to maintain (ok not by much).
On a car like this, why do you need it? Power steering takes away some of the feel. Look at the Elise and similar . .

Not sure I can comment on the Sierra as a donor. If you are not using box or engine, which bits are you going to use? prop?

Don't want to use power steering - but if my donor has it can I just use the steering system without it? Or do I need to make sure my donor doesn't have it..

As for the donor, I'll use diff and rear axles, front steering, hubs, brakes etc.

Probably only need to change the rack, I can't imagine Ford would have had two different front end geometeries just for PAS. Have to change the rack 'cos heavy resistance to turning wheel on PAS car without the PAS pump running. The steering ratio is usually higher on PAS cars BTW, might be worth checking on how many turns lock to lock needed on PAS and standard versions before you bin the PAS
John

I used the estate, (ok so it was a 4x4) but the steering column is the same but I dumped the ABS and power steering. Used the parts from a standard Sierra. All parts fitted fine.