not as far as I know, but I wouldn't be surprised if there is, I just haven't searched for that
ryzen really likes fast ram, certain cpu operations directly tied to ram speed.
You can get any 2x4 RAM kit that is 3000MHz. The Team Vulcan one was the cheapest 2x4 3000MHz kit I saw listed in the US, pricing can change from country to country
And yes, there is a difference. 2x4 RAM is typically installed in dual channel configuration (alternating slots, you can check the motherboard manual to see specific slot configurations). Dual channel RAM yields a performance increase (especially on AMD Ryzen), and if you get a 3000MHz kit (which is supposed to stable when OC'ed to 3000MHz) then you should notice a good performance increase as well.
No problem!
It does look like the SSD I picked isn't available, but that could change based on which European country you're in as well. There are also cheaper SSDs, Kingston, and Sandisk have some cheap 250GB offerings if you wanted to look at those instead.
PCPartPicker part list / Price breakdown by merchant
Type
Item
Price
CPU
AMD - Ryzen 3 2200G 3.5 GHz Quad-Core Processor
€97.90 @ Alternate
Motherboard
MSI - B450 TOMAHAWK ATX AM4 Motherboard
€107.91 @ Mindfactory
Memory
Team - Vulcan 8 GB (2 x 4 GB) DDR4-3000 Memory
Storage
Western Digital - Blue 250 GB 2.5" Solid State Drive
€46.49 @ Amazon Deutschland
Storage
Western Digital - Caviar Blue 1 TB 3.5" 7200RPM Internal Hard Drive
€44.90 @ Caseking
Video Card
Sapphire - Radeon RX 570 8 GB PULSE Video Card
€107.04 @ Amazon Deutschland
Case
Thermaltake - Versa H21 ATX Mid Tower Case
€33.70 @ Amazon Deutschland
Power Supply
be quiet! - Pure Power 11 CM 500 W 80+ Gold Certified Semi-Modular ATX Power Supply
€71.90 @ Amazon Deutschland
Prices include shipping, taxes, rebates, and discounts
Total
€509.84
Generated by PCPartPicker 2019-02-11 19:09 CET+0100
Is just a tiny bit overbudget (subject to change with sales and availability as well), but should be fairly decent and will provide a pretty good upgrade path.
You don't need fancy keyboards or mice, you could just get the cheapest you can find. You don't need to buy a Windows key, it will function quite well without being activated. If you don't activate Windows, then you just have a watermark in the bottom right and can't use customization options (good trade-off if you ask me); and you can always purchase a key later (or find one cheap somewhere).
Edit: I also highly recommend getting an SSD, if only for your boot drive. The speed increase is definitely worth it.
Edit 2: Get a 1080p monitor, they are fairly cheap and are good for budget/low end gaming.