No bridesmaids

As I was reading this I was initially like "oh no, don't ask them to do that" until you said you'd be contributing towards their dresses - I think this makes it OK and a nice gesture. Be aware that a lot of wedding guest dresses are closer to £100 so you might get some push back if your girls were unlikely to buy a new dress for your wedding until you mentioned this, it may leave one girl feeling left of our something. If you could swing it, perhaps offer to pay for the dress outright as long as it's not more than X amount but maybe that should be a little higher than your original budget. I know weddings are horribly expensive as is, but thought I'd mention as I have a few weddings coming up and the cheapest dress I've managed to find that feels appropriate for cocktail/semi formal is £55 (and that's for one with a disco theme), the others are all £70-100 from high street brands.

Really nice idea to gift them flowers for their hair too

I definitely think that when you ask them, you should tell them that they would have been bridesmaids and they only reason they are not is because you're only having your sister as MOH - otherwise they may feel like others were chosen as bridesmaids and they just didn't make the cut!

I think it sounds fine - you are giving them a wide range of colours and unlimited style options, so they can pick a dress they can wear again and again, and you're also offering to put some money towards it. They can always say no if they don't like the idea! As Chloe says, I'd explain to them that you would have loved to have them as bridesmaids but couldn't manage 6 in addition to your MOH and couldn't choose between them.

I can understand why you won't want to add an extra 6 people into the room while you are getting ready!