When I started, I started with Slimlines because of the cost. They were on sale at the time for about a dollar and a half. After only a couple of pens though I bought the larger Comfort center bands and a set of Comfort bushings so I could leave a little more blank on the tube without making them look so pregnant. My next venture was to Cigar style which is remains one of my favorite styles to date. The third was Euro styles like the PSI Designer series which also remains one of my favorite pens. I also followed with a lot of Sierras, Bolts, Jr. Gents, and a handful of "Themed" kits.
Knowing what I know now, my recommendation to a new pen maker would be an Alumilite blank because of how nicely Alumilite turns compared with the other acrylic (i.e. plastic) blanks and it avoids the need for developing some kind of wood finishing regimen.
As for the kit, I would recommend a single tube variety like the Stratus, Executive, Cosmopolitan, DuraClick EDC or Sierra/Gatsby type so there is only one tube/blank to contend with. However, I would recommend making the decision based on subjective reasons - not based strictly on cost or ease of assembly as I think it is just as important to build a style that they would be most happy with carrying around in their pocket.
Of course my oldest son had some guidance, but his first pen was a Acrylester blank using a Classic Elite 2 fountain pen kit. Certainly not the easiest turn, but he took his time with turning and it turned out great. I think all new turners kind of need to feel their way through things. For me, the satisfaction of making something to be proud of and enjoying the experience is priceless.
Regards,
Dave