Other than being the least expensive kits, there is no reason to start with slimlines or any other kit for that matter. They are all made basically the same way....drill, glue in tubes, square the ends, place on mandrel with bushings, turn to the proper diameter, sand, finish, and assemble. A few kits require turning a tenon for the CB. I would say to browse the catalogs or websites of kit sellers, find a pen you like, read the instructions to see how the pen is made, order one and get after. Pick a kit that you feel confident in making. Most twist pens that use a parker style refill are a bit sensitive to barrel lengths but follow instructions and you will be fine. I think the slimline has garnered the reputation as the starter kit because when many of us started there wasn't many choices and when a few of us older ones started they were the only kit, right Ed?
Do a good turn daily!
Don