Hi Cole,
To start with as you are new to composites, and please don't take this the wrong way but walk before you can run.
1) Not sure what you mean by inflated prices, they are usually about the same within a couple of £/m, having an amarid/kevlar hybrid weave as backing layer will help prevent carbon shards going everywhere in a crash, some like using diolin as a true backing layer as it tends to stretch more on impact and helps keep the laminate together. If you do crash, the section with fail and is bin fodder anyway so you are preventing secondary damage to occupants or parts rather.
2) The pattern is just the weave pattern so it can be twill, plain, fish, satin etc... so whatever you prefer really, twill will drape better over curves than plain, 2x2 twill is the most common, so there are two tows at 90deg to every tow at 0deg in both directions hence the pattern. Plain woven is an over under patten, satin & fish are not worth thinking about in your application.
2k/3k/6k/12k are the number carbon strands per tow and has an effect on the look and the number of tows per inch/cm as this will reflect the density of tows better.
3) Any core whether it's PVC, Corecell, Soric, balsawood, cork etc... is used to space out the top and bottom of the laminate increasing it's stiffness (moment of inertia or second moment of area) without adding too much weight or the additional cost the fabic/resin to make the core. Use it to stiffen areas or entire panels where required. Any fixing locations need to be solid rather than with a core so these areas can be locally infilled, you make the laminate as thick or as thin as you need for the specific area.
4) before you blitz a grand or two so on a roll from the other side of the world without knowing what you want get some samples from a UK supplier, EC do samples about 50p each so get a few before you buy so you know what you want. The weight of the material per square metre is exactly that, 200gsm is a nice fine weave and lightweight fabric, 375/450 is a bit heavier and typically is a larger weave, 650gsm is a heavyweight reinforcement used mainly for structural and backing so you use say 200gsm as the layer you'll see and then 650 or whatever behind that to add the strength and stiffness.
As you are using it cosmetically then the weave and weight are really for asthetic only so it can be whatever you want
https://www.easycomposites.co.uk/composite-reinforcement-samplesBut seriously, if you have never worked with the stuff before, buy a few metres and have a play. Also don't skimp on the resin.