Adjustable Linkage? (Purpose?)
Yes, you need this to set your axles square and to adjust pinion angles.
Honestly, there is no average. Look harder at "value" for your dollar. I like Rock Krawler long arm systems.
Difficulty to install (mechanic but never installed a lift before)
In most cases, you will need to know how to weld, comprehend instructions (sometimes vague ones) and understand suspension geometry.
How do the long arms connect to the frame? (welded supports or does kit come with bolt on parts?)
Some have brackets that weld to the frame, some have a new center skid that the arms connect to. In the end, most are tied to the frame and center skid.
Will i need a Welder?
In most cases, yes. You will also need to be equipped to remove the stock control arm brackets from the frame and possibly rear axle.
How long will my Vehicle be MIA?
Depends on the kit you get. If you are very proficient and well equipped this may be done in a long weekend.
Will it affect Handling on highway? (Compared to Short arm Susp.)
I liked mine. My TJ I set up on long arms, my LJ I have on short arms. The higher you go, the larger the difference you will notice between long and short arms. I had the RK X-factor 4" long arm system and the ride and handling was amazing. I set my LJ up differently because it is used differently than my TJ was. A 3"-4" short arm system will drive and handle very well too.