Hi GPA, my experience is limited, however, I'll give you what I know. As for the bearings, in my experience you probably would not notice a difference, and some downhill riders prefer better quality bearings with a bigger tollerence (lower ABEC number) because they are less likely to sieze at high speeds. Go for the reccomendations of a good skate shop like hopkin.com.au/ good guys, all skaters and some of the countries best racers. Buy a quality brand downhill specific bearing.
Same with the bushings, you might even end up with a combination of different durometers, I run three different grades on one of my boards. Venom bushings are really good, and at 80kg, I have not yet blown out bushings. Get the right feel and don't over or under tighten. Check out the buyers guide hopkin.com.au/longboard-buyers-guide.html for some good tips on board set up.
Wheels - a whole different topic. For sliding go harder. For grip and drive go softer.
For acceleration go smaller, for ride comfort go larger. Obviousley you have to compromise something. Check out the downhill race wheels that the podium winners are using.
Hope this all helps, but you'd still gain a lot from talking to some real experts.

-user-u

Thanks guys - just came back from 1 1/2hrs of longboarding... it's a Public Holiday here so we headed to the new [Greenwood] Train Station car park which is a good size and the whole thing on a good slope - and it's that new hotmix bitumen, so it is nice and smooth.

We did a speed test. Standing start - wheels to the curb, no push 50m straight run.

We are both on 70mm wheels - I'm on 78a and ABEC 9 and my son's on 83a and ABEC 7. I have a 20kg + weight 'advantage'.... he was 4-5m ahead of me at the finish and continuing to gain distance ever so slightly. Did the test twice over... I reckon it was the harder compound wheels for the win - but interesting to note that the 9's did not outrun the 7's [but they sure spin a lot longer].

PS - that guy does a whole series of YouTube videos and they are all pretty good. Particularly for teaching a 14yo... [who won't listen to Dad but will sit in front of YouTube ]