You may or may not know that part of the MRC clubs membership entitlement.
Over the years I have carried out many valuations, with one letter for the insurance company and one for the owners file.
First of all many cars actually need to have an individual assessment made due to either the rarity or the extensive modifications that have been carried out.
First and foremost, whist market value has an influence, all insurance valuations are based on a replacement value should the car be a total loss.
I would normally ask for detailed pictures for file plus a list of mods ( I have had insurance company's question valuations)
Next question would be, are there bills to back up the cost of modifications should the need arise ?, and then obviously the overall condition of the car would be taken into account, then lastly I ask the owner what they think its worth and most end up undervaluing their own car !.
Sadly over the years I have seen many who don't bother with a valuation and then just get paid market value and end up just about nothing !