If I were you I'd save up a bit more and try to get a ~£900-1000 budget which allows a much better flexibility when it comes to parts and you won't have to cheap out on certain parts (usually the motherboard or the PSU become the victims of this).
In my opinion it's really hard to optimize a rig for Arma 3 as Arma 3 itself is an unoptimized game, if you like playing other games as well then I'd certainly not use Arma 3 as the benchmark. This is also important with your CPU choices, AMD Ryzen provides a much better value when it comes to an all-rounder CPU whilst Intel beats AMD when it comes to strict gaming performance due to better single core performance (Intel CPUs can achieve higher core clocks but AMD is slowly closing this gap).
Also bear in mind that AMD CPUs come with really good stock coolers that allow slight overclocking whilst for Intel you need third-party coolers, especially if you go for an unlocked chip to overclock and for that you need a more expensive motherboard as well. I actually struggled to find a good no-compromises Intel system for 1000 quids.
As of SSDs a 256GB is a minimum, currently my Arma 3 folder is 123GB and I only have 3CB mods (including Unsung and WWII) and as I play other games I have to keep my Arma 3 stuff on my HDD. I'd say a 512GB would be the optimal for Arma as it gives you some room to expand in case we add more mods and you can install another 1-2 favorite games on it for better loading times. Thankfully SSD prices are pretty good nowadays and you can get some awesome new 500-512GB SATA3 SSDs for around £70 but the much faster NVMe m.2 SSDs aren't much more expensive either.
If you have a bit of increased budget I'd recommend similar builds like the ones below:
- The one with AMD Ryzen as an all-rounder bang for the buck: https://uk.pcpartpicker.com/list/h3FkP3
- The one with Intel where single core gaming performance is above all: https://uk.pcpartpicker.com/list/9KXQ9J
Going for a pre-built instead is a viable option because it saves you the hassle of parting out your system, doing compatibility and system checks. Personally, I think building your own PC is better and the experience of actually building it is really fun but obviously there are others who don't find pleasure in such things and just want to find something that works.
What you need to know about pre-builts and the companies making them is that many are using proprietary components that can make even basic upgrades or fixes impossible, forcing you to mail everything back to the company. LinusTechTips has just recently started a new series about these pre-builts and the customer service / support behind them, pointing out how most of these companies are treating their potential / actual buyers, I do recommend watching the videos in the series, it might help you make a final decision:
Part 1 (Buying)
Part 2 (Unboxing)
Part 3 (Customer support)
Edit: I run Arma 3 on Ultra settings and get 20-40FPS (avg. 25-30) on our OPs. I have an i5 3570k, 8GB DDR3 and a GTX 1060 6GB.