When choosing between kits the main questions are usually what are you getting for your money? how long do you plan to keep the car for? and how do you intend to use it?
If you are looking short term then there are savings to be made by choosing a cheaper kit though you need to offset this to some degree by the residual value of the top end kit.
If your plans are longer term then the kit with the highest longevity will be a better choice financially as you are unlikely to incur service costs whereas the cheaper kit will need new bearings at some point, fitting and a fresh geo which will probably make it the more expensive of the options.
You also get all the benefits of the highest specification aluminum alloys and custom designed bearings, i.e the kit is the lightest for its strength, ultimately the strongest, the stiffest and as mentioned easily the most durable backed up with a 4-year warranty on the bearings though you can expect 8 years as a minimum. The rest of the kit will last the life of the car.
In answer to your other questions the Spitfire kits are fully assembled, orientated and set for bump steer, the instructions are clear and comprehensive and also include a tool list, as you have already mentioned the bolt torque I would consider you know what you are doing and would have no issues with fitting the kit yourself.
Other advantages are the optional components which are designed to work with the kits i.e for the XV the brace option, this helps to avoid the fractures seen on some cars with a track/heavy use history as well as tightening the rear of the car nicely.
Once fitted the car should have a full geo to get the best handling characteristics.