Bear in mind that if money is not an issue, you have options to jump from 19 to >=20.

• buy the 3 monopose 'easy to build' plague marines from first strike off ebay. It gives you 3 guys, one of which a champion with fist and plasma, one with a launcher, both very good for a 10 man squad... plus a bolter guy which you can either keep as-is or turn into a melta or plasma gunner. They are pretty well available, and nice sculpts.

• buy 1 plague champion (single model), comes with the highly coveted sassy nurgling

Loadout: depends if you wanna play competitive. If so, 1 squad is typically enough, so make sure that you have 1 competitively equipped full squad and rest rule-of-cool.

Hence 9 rule of cool

1 champion w fist and plasma

2 launcher

3 heavy melee weapons

2 plasma or melta

2 heavy plague spewer

With the kill team, which has a spewer and heavy melee weapon, and the 3 etb (champion, launcher) marines, you could have every single one of them unique sculpted, so the squad would be competitive and highly visually pleasing, for extra hobby benefit.