Box design is decided upon after knowing what type of system it is to be used with. i.e. 3 or 4 way system. active or passive. large or compact. acuracy or sheer pressure. etc etc.
I agree with planet10 that for you money 12" drivers offer all round value, and in the right enclosure can hit lower and harder than 18,s.
Try coupled cavity enclosures for example using a driver with a strong motor assembly. Fane are awesome drivers as are Precision devices but can be pricey new. for 80 quid you should be able to get a v.good 12 driver and try modelling a few using winisd