I think it's personal preference. I'm going to have an upstand and a splashback. I'll try and attach a pic so you can see what it would look like (this is a mirrored splashback rather than glass though)

Look at Earthstone as a quartz alternative. It’s a 6mm solid surface wrapped around a chipboard core, so it has most of the advantages of a solid/quartz surface at a fraction of the price. It can be shaped and sanded, left matte (which is what I did) or polished to a high gloss. I’m really, really impressed with mine—cost around £1300 for two long lengths plus breakfast bar AND delivery to Northern Ireland. Looks like Worktops Express don’t do it anymore, but available elsewhere.

That looks a very well built kitchen so ripe for updating. The kitchen looks more like cream than magnolia, not that it matters! I think your new colour scheme looks great, modern but with classic styling. I think I can see a deep navy/grey in that beautiful worktop? I agree with Lacuna to get test pots, paint them on a piece of A4 card and try them next to the worktop. I would do that at various spots in the kitchen. As to the wooden worktop, you can sand it by hand and then apply several coats of Osmo Top Oil which is made specifically for worktops. I use it on my oak worktop.

Corian is the same as Solid Surface but a brand name. Both fabricated in the same way but a huge difference in the price. Quartz as a material is the same as granite. Quartz probably cheaper than granite. If you want something similar to Carerra Marble then look at Mirostone Taj Mahal. It all depends what area you live in for recommendations.