be careful with gtx magnatec, its one of them oils that got popular through massive advertising campaign/marketing etc back in the day(which worked)
but isnt very good oil for ek9, breaks down easily and not that good at taking high temps
as for first post, opies oils is best place to buy, you dont have to go for the top of range motul or silkolene although if tracked you should but this motul will be fine for you and near your budget (also 5l so leaves 1l for topup is you burn oil)
Motul 8100 X-clean 5W-40 ACEA C3 Gasoline & Diesel lubricant for Euro IV engines
or if you are gonna go cheap route (not recommended with oil) at least get something like this
Motul 6100 Synergie+ 10W-40 Gasoline and Diesel engine oil Technosynthese