Are all newish hotpoint washer/dryer model parts compatible?

I have a hotpoint wd640P that has been diagnosed by a washing machine guy from the yellow pages as a dead PCB (had the dreaded F08 code, and some others), he said it is not financially worth fixing (the machine is 18 months old!!)
A friend has offered us her Hotpoint WD420 with a burnt out motor.

We would love to get our machine fixed under the 5-year-parts guarantee, but my husband was made redundant last year, and I can only work part time due to ill health, so finding the £100 is not an option unfortunately.

So, a couple of questions for you please
1. Are the parts cross compatible?
2. Is it worth robbing the functional part of one machine to repair the other?
3. Which machine would you salvage?

I would appreciate any input, as I have been treking up to my Mums with bags of clothes everyday for 3 weeks now, and with a four-year-old muck-magnet child, the washing mountain is growing!

Thanks for reading