Suit yourself. Maybe keep Sideswipe in car mode, and get HA Jazz to be your "silver standing car robot"?  

 

Really? thats dope. Can you buy it only as a set with buster prime? Or can you buy it separately?

 

Optimus Prime: RotF Leader (either version)
Bumblebee: Human Alliance
Jazz: Human Alliance
Ironhide: Movie Voyager or Premium Voyager
Ratchet: Movie Voyager or Best Buy Premium Voyager
Arcee: Deluxe
Chromia: Deluxe
Wheelie: Legends
Sideswipe: Deluxe
Jolt: Deluxe (not much to choose from so far though)
Skids: Human Alliance
Mudflap: Human Alliance
Jetfire: Hasbro RotF Leader

Megatron (first film): Premium Leader
Megatron (second film): RotF Leader
Frenzy: Human Alliance
Scorponok: Movie Deluxe
Barricade: Premium Deluxe
Bonecrusher: Deluxe (also not much to choose from, sadly)
Blackout: Premium Deluxe
Starscream: Deep Space/Premium Voyager
Brawl: Movie Leader
Devastator: Legends combiner
Rampage: Takara red repaint
Demolishor: Takara white repaint (the normal Hasbro version can be repurposed into Scavenger)
Long Haul: Voyager
Ravage: normal Hasbro Deluxe
Soundwave: Legends
The Fallen: normal Voyager

Too little variety for Sideways, Mixmaster or Grindor to choose anything beyond their 'normal' releases.

 

Also, I read on tfwiki that the Buster Prime that comes with the Jetfire varies a little bit from the Buster Prime that comes by himself. What are the differences?

 

The Buster Prime that comes in the two pack with Jetfire has the face-plate head, like the regular Leader Prime, the solo package Buster Prime has a face-plate-LESS head, has a molded mouth instead of a face-plate.

IMO, the best versions of each character are as such: (most of these I don't have, just going by what I've seen recommened by Peaugh and others on youtube)

Prime: RoTF Leader (either version you can get hold of)
Jetfire: Leader
Megs: I'd say either the Shadow Command (apparently he has a more acurate head, and better paint) or wait and see what they do for TF3
Barricade: Human Alliance hands down. It's probably the best barricade mold we'll ever get. (I'm definately gonna get him when he's released in the UK)
BB: Human Alliance. Again, one of the best (if not THE best) BB molds we'll ever get.
Ratchet: I'd most likely say Voyager class.
Ironhide: Perhaps the first movie Voyager, or the second one, not sure.
Starscream: Wait for Leader.
Skids, Mudflap and Jazz: Get the HA versions, the HA version of Jazz probably won't be released until round about christmas time.
Mixmaster: Voyager version.
The Fallen: Robot Replica. Yes, it doesn't transform, but I've heard that it's the best mold of the character there is, unless they do a Leader size version, which they probably won't.
Arcee: First movie Deluxe. The RoTF version just looks awful, and the HA version is a repainted HA Chromia mold (they mixed up the colours for the HA bikes)
Blackout/Grindor: Voyager class.
Devastator: Legends version.
Sideswipe: I'd say go for the Deluxe, HA has more detail, but he's got ugly amounts of back-kibble that makes his robot-mode look really bad IMO.
Brawl: First movie Leader.