It is lovely. Grand Monastery is a fantastic place - I had a tour of it a month or so ago and the spa looks amazing. Same problem as Malina in that it is not really walkable to the bars and restaurants from town but that is why there is so much on site for you to enjoy.
It is busier half term week but not unbearably so. Unlike Bankso there is no gondola so not so much of a bottle neck to get up the mountain in the morning. However, the flight prices are pretty astronomical half term week as well as apartment availability reducing rapidly. If you can go outside of half term then you would get a much better priced holiday.
Can you fly from Stansted and do Ryan Air flights to Plovdiv??
Just done a quick trawl. Sat 19th to Sat 26th - half term week, flights are currently £340 return per person.
The week before half term: Sat 12th to Sat 19th are under £60 return.
And they wonder why we pull the kids out of schools term time! I know its basic economics but still, when you are a family of 5 every penny counts.