Are you sure 450W will be enough if I want to upgrade in the future? And I've heard it's best to only use 2/3 of the PSU capabilities max? And if I want to upgrade to a RTX 2080 and i9 9900 sometime won't I get problems with my wattage overhead? (if that's the right term)
Whoever said that you should load the PSU at max (insert percentage below 100 here) is absolutely clueless. Are you going to upgrade to an €800 GPU? Don't base your PSU choice on some arbitrary non realistic criterion.
If you actually are planning on upgrading, just get the Whisper M 550W or Straight Power 11 550W. The RM550x has two PCIe connectors on a single cable, so I recommend not using it for GPUs with a TDP above 225W.