bruh, the only reason i have the gt1030 is because my 10 year old prebuilt was tight on space and thats why i chose that, i am not gonna buy a new gpu, i just asked for an upgrade for everything but my gpu ans stoarge
Maybe someone here with more building experience will disprove me, but under 300 eur is going to be challenging, especially if you want to ensure future upgradability, all you have is a GPU and storage, and you want new.
Edit: somehow missed the part about where you want to find the parts. I'll take a look at that website, but still it seems like a stretch.
Edit2: found something that should work, by picking the cheapest components I've actually heard of (e.g. some coolers, cases, PSUs on that site I've never even heard of). I'll list the parts I've found and the total price, which is under 300 eur.
Caveats: (1) No idea if this will manage your workload. May be a good idea to upgrade RAM if it's within your budget. (2) I don't know for sure if everything will fit together properly, but should: I usually do preliminary checking on pcpartpicker which tends to point out most of the incompatibilities in dimensions, power draw, etc. (3) It's a microATX build. (4) The motherboard supports Ryzen 5, but needs a bios update in order to do so if it is running an older bios. If I recall correctly, you need to actually have a compatible CPU installed in the motherboard to do the upgrade. So it's worth checking with the seller what bios version they have, or making sure you have a CPU you can use for this.
MOTHERBOARD: Gigabyte B450 Gaming, PRODUCT CODE: B450MGAMING, PRICE: 78.25 €
CPU: AMD Ryzen 5 4500, PRODUCT CODE: 100-100000644BOX, PRICE: 79.65 €
COOLER: Cooler Master Hyper H410R RGB, PRODUCT CODE: AWCLMWP00000023, PRICE: 28.60
RAM: G.SKILL Aegis 2x4GB 2400 MHz, PRODUCT CODE: F4-2400C17D-8GIS, PRICE: 22.25 €
CASE: Deepcool Wave V2, PRODUCT CODE: KODCOOE0WAVEV20, PRICE: 29.90 €
PSU: Deepcool DA500 500 W, PRODUCT CODE: DP-BZ-DA500N, PRICE: 38.90 €
TOTAL PRICE: 277.55 €