There's plenty of threads on this in this section of the forum... So if you read through the titles over the first few pages for appropriate threads, you'll probably find lots of advice already.
You need 2 Troops and an HQ, so generally you'll need 2 boxes of Sisters and a Canoness blister to get started. On the limited budget, why not just pop for 1 Sisters box, and don't buy anything else until that is painted - you can judge how you like painting them, and if you'll be able to "hang" with doing a whole army of them, AND you don't get a whole pile of unpainted tin laughing up at you, intimidating you.
You need 2 Troops and an HQ, so generally you'll need 2 boxes of Sisters and a Canoness blister to get started. On the limited budget, why not just pop for 1 Sisters box, and don't buy anything else until that is painted - you can judge how you like painting them, and if you'll be able to "hang" with doing a whole army of them, AND you don't get a whole pile of unpainted tin laughing up at you, intimidating you.