You really ought to think about what brews you want to make first.
If you want to make nice summery pale beers, then some Pale Malt or Lager Malt would be a good starting point, adding a bit of Wheat in for head retention, and maybe some CaraMalt to add a little body and sweetness.
If you want to make a wheat beer though, you will need lots of wheat malt.
If its Darker Beers you are after then some chocolate Malt, Amber Malt, and maybe some Black Malt as well, you can drop the Lager Malt if it's darker beers you are after.
I would recommend working out some recipes as to what you think you will be brewing and just buying the required ingredients for them. Over time your stocks of the specialist Malts will build up, especially things like Black Malt as you use very little of that.
I buy all my ingredients from the Malt Miller as his service is second to none, but there are other suppliers at the top of the page who all have a good reputation.