We'd recommend an E93 generation BMW 3 Series Convertible (launched in 2007). It strikes a good balance between old and new, with plenty available on the used market and a clever electric hard-top roof (ideal for daily driving). A budget of around £6000 will get you a 330i, or closer to £9000 for a desirable 335i. You could also look at the older E46 but look out for rust - many are starting to look quite rough.