Whether your kit mandolin would be worth $600 or more really depends on how well you put it all together. A big factor in the equation is how well you can graduate the plates, carve the tone bars, tune the top and execute the bindings and finish. If you get that right, you should have a very good sounding instrument. Since the kits are all machined to fit, things should go together pretty well.
As for tools, you will need chisels, gouges, maybe some finger planes, a router table, a Dremel tool and base, sandpaper, a coping saw or jig saw, at least a hand drill, finishing materials, various adhesives, and a bunch of clamps. Seems like there is always another tool or accesory to purchase once you get going.
My advice is this: if you have an interest in building and are passionate about it, do the kit. If all you are looking for is a better mandolin, save your money and just buy a better instrument. In other words, if you only plan to build one, you may be better off just buying an instrument. If, on the other hand, you envision making a number of mandolins in the future, the kit will get you started down the road.
Wisdom comes with age, but sometimes age comes alone