Rendering a newly built concrete block wall

I'll try to answer some of your questions.

Is your area subject to rising damp and, if so, have the walls got a damp course? If you're not likely to get rising damp or you have a damp course, then there's not much to bother about. If you do have rising damp and no damp course, then you need the final surface to be breathable.

The surface of the walls appears to be rough enough that you can apply render direct to them without a sealing coat. One problem with Spanish builders is they they apply render direct to a wall that's bone dry. The wall sucks all the moisture out of the render, so it doesn't bond properly, and later shells off. Give the walls a wetting before you apply render.

If you want to play safe, you can seal the walls with a mix of one part PVA adhesive to four or five parts water, and this will help the render to bond.

I'm not a fan of ordinary sand and cement render here, as Spanish sand is very coarse which, in my opinion, makes the render difficult to use. I've done it a few times, but don't like it. What I prefer is what they call 'monocapa' - a one coat mix which is easy to apply. There are a lot of makes, but I don't know which is best. For examples, there are Gecol and Morcemdur Raspado. The latter can be had in two varieties - one you can apply thickly as a base coat, and a fino to use as a top coat. You can also get different colours, so could avoid having to paint it.

I hate plastering and rendering, as I've not enough experience to be able to get a perfectly flat surface. I do the best I can, wait until it's going off, then rub it over with a damp cloth or sponge

If you do paint the render, then I'd definitely seal it with the PVA mix I referred to above.

I've used Adoral Monocapa from Brochacolor to paint the outside of the house, and have no problems with it. There's a paint shop in Quesada that sell Dulux, but I don't know if they stock Weathershield.

Depends what you mean by breeze blocks. I've not come across the lightweight ones here, and have used either ordinary concrete blocks like the ones in your walls or terracotta blocks. I must admit that I'm a fan of terracotta blocks. They come in a variety of shapes and sizes, are lightweight and easy to handle, and are really strong. For example, you could use what they call 'baldos' (long thin terracotta sheets in a variety of sizes) for the top of your wall instead of the much more expensive capping stones.

Thank you for all this info, very helpful. 

I’m also making the area directly in front of the wall a raised bed for plants etc. So will probably need to use the PVA as a sealant also on that. 

Wall doesn’t have a damp course. 

PVA doesn't actually seal it in the sense of waterproofing it. It just stops it being too absorbent and the adhesive helps the bond.

For proper waterproofing, there are various products you can buy. The easiest is liquid bitumen, or you could go for a tanking compound like Weberdry 110FX. You can even get a liquid you add to mortar or render mix to make it waterproof, but I don't know whether this would be effective with the coarse sand that's used here.