I think Remus' list is great, it could maybe be 1 white, 1-2 black (or navy), 1 nude, 1 pretty/fun for a start point: as quality bras for a larger bust will likely be in the 30-40£ range - so buying some now & some on sale
Second the idea to research your styles - bravissimo, HOF/Debenhams/JL for a fitting & try styles. Then get the bra name & look it up online to get a good price (eg last year's colour etc...)
You can save a LOT on lingerie buy buying online/sale time/clearance/20% code etc - figleaves.com has a huge selection esp of DD+ bras (also has outlet, and good january sales if you want to start with a capsule & buy more later - often have 20% off weekends eg today). I have often bought 45£ bras for eg £15-20 at dept store clearance section (charnos, fantasie & freya at HOF recently)
Great brands are fantasie, freya,panache, curvy kate, charnos & bravissimo own label - (last 2 are better priced). I'm juding based on shape & durability (these brands would last me 2-3 years in a rotation, M&S or JL own label generally 1). If you try enough bras on you will work out what you like. Not a huge fan of M&S but some of DD range can be worth a try (esp if not the bras you will wear most)
I think you need 2-3 pairs of undies per bra, the matching underwear seems pricey but depending on the brand can wash & last really well (fantasie elodie look brand new after a year). Otherwise if you are just doing a colour match M&S online has reviews of how they have washed/lasted - useful if you are buying lots at once - and VERY variable
If you have at least 4-5 good bras you can make them last by: never wearing 2 days in a row (elastic needs to 'rest' so leave to air after wearing), getting several wears before washing (its what wears them out), and handwashing + air drying (less tedious if several bras at once which is why good to have lots) - go easy on the fabric conditioner too