I'll get him the crusader variant,
and some small magnets so that you can swap the weapons to a Redeemer if you choose!
for the troops would 20 marines for 2 troop choices be enough
At 1500 it's plenty, but you wouldn't be hurt from having a third.
If you're planning on going with a chapter out of the SM codex, I would suggest having a solid read over the Chapter Tactics and see which one is the most interesting. That will very much effect how the models will be assembled (in terms of special weapons) and what types of units will be bought. For example, my BA army gets all of my assault marines and my IF army gets all of my tactical marines because for BA assault marines are Troop choices and in my IF army all bolters benefit from Bolter Drill. Likewise, I don't have a devastator squad in my BA army because it's too stagnant a unit but do play two of them with the IF due to their sweet Siege Masters rule.