Hi Robin
I agree with Glenn's points. Buying a computer can be a bit stressing as there is a lot of salesman flimflam about whether you need this or that, unless you know exactly what you need. The key thing is to find someone you can trust, if possible.
My last computer was purchased from an online store (IJK in Queensland) where you can custom build the system to your specifications. There are several companies that do this, but you need to know what you are doing to get the right mix of things so this can be daunting.
Brand name desktop computers (eg Acer, HP) from places like Hardly Normal can work fine, although they can be overpriced compared to custom systems. I would always go for a tower type system (which can still be reasonably compact) rather than an all in one system so that you can change the computer separately from the screen at some future point if you want to.
Just like cameras, the key is 'the best you can afford'. I would suggest for raw processing you should try for an Intel i7 or similar, at least 16 G of RAM, and as Glenn suggests an solid state drive (SDD) on which you put the operating system, together with a 2TB minimum 'normal' hard drive for storage. I recently added additional RAM to my computer (to 24 G) but it did not make much difference because in my case my processor (intel i5) is the limiting factor. It is frequently working at 100% of capacity. SDDs are essential for Windows 10 in my opinion, as they load MUCH faster, and I have upgraded all computers we own to SDD for this reason. Going from a 'spinning' hard drive to an SDD is itself equivalent to a computer upgrade.
You do not need a special add-on video card no matter what the salesman may say. The 'on board' video that comes with virtually all modern computers (which is built in to the main processor board) is sufficient. Specialist (expensive) video cards are designed for computer games, which are highly demanding of video capability, and not needed unless you are a fairly high end gamer.
Avoid using laptops unless you really need the portability. Mid range laptops often have good specs on paper but these are slowed down in practice by the power management technology (aimed at making the battery last), so they often don't run as fast as the specifications would suggest. My i5 desktop is for most purposes faster than our mid range i7 laptop.
Also consider your screen needs. A good screen with well calibrated colour is very helpful in raw processing, with a minimum specification of 'full high definition' (1080 by 1920 pixels) or higher.
Cheers
Simon