wanting to sell but render quotes too expensive.

Beenie

This is an impossible question to answer.

There are people like me who run a mile if they see houses 'done up to sell' as it usually means the cheapest units, bathroom figments and decorating jobs. I'd rather do that sort of improvement myself.

On the other hand, there are people who love a ready-made house and will be happy to pay, especially if it's in a good area.

You must therefore do what you can afford and are happy with. There will be a buyer out there whatever the condition of the house- as long as the asking price matches.

road2manchester

sound expensive, can you load pictures. If the walls are damp then cement based renders are likely to come off due to it not being breathable. Average for silicone renders is 65-75£ sq mtr all in (except scaffolding and skip).
Decopierre, K-Rend, Webber are more expensive alternatives but a better finish that don't need painting so enhance the kerb appeal.