Hello Florian

Firstly, the Jelco 750D is a fine arm and suits the Technics very well indeed. Having heard Marco's (AoS owner) deck, I can attest to that. I doubt that the 309 will better it in that application, although I have not heard one on a Technics. I used to have an SME IV and I don't think that betters a 750D as a cartridge platform.

As for power supplies and upgrade order, you will find days of reading material in this forum. Briefly, there are some upgrades that are mandatory to start your true high end Techie experience. I would certainly look at the PSU next, since you are already considering the arm. Having heard a few I can only recommend the Paul Hynes SR-5 as being superb both for the original bearing & platter and being able to handle bearing/platter upgrades into the future.

You need to follow an approximate upgrade path of:
feet > arm > PSU > mat > bearing > platter

For the feet, I don't know of anything to touch the Isonoes. I also strongly recommend a weight or clamp and a good mat.

Good luck and keep asking questions.