In general, it's better to have more troops (choices) than not. This applies to all armies, not just marines.
This thread discusses how many troops to take in what size game.
http://www.dakkadakka.com/dakkaforum/posts/list/253983.page
Besides more troops, think about what kind of wargear you'd like on your units.
Also, Landspeeders are fun, but not the most durable unit in the codex.
This article can give a good idea how each unit in the codex is usefull.
http://www.dakkadakka.com/wiki/en/Space_Marine_Tactica_%28Age_of_Egos%29
Good luck, and don't rule out starting a non-marine army...