It would probably be cheaper in the long run to sell the Roadsport and buy an R300 or R400. It depends how attached you are to your current car and what condition it is in versus the likely condition of an R300/R400 within your budget. Also would you use Caterham for upgrades or a 3rd party (DVA, Hellier, etc.)? An upgraded car is never worth as much as the "proper" version - it would always be an upgraded Roadsport and worth little more than a standard Roadsport.
Having just completely rebuilt my 1997 Superlight I am sure that I could have sold it and with what I spent on the rebuild plus what I would have got for the car could have bought an SLR or maybe even an early R400 (hell I could have bought that R500 going for less than £20k )...But I now have an as new chassis, it's "my car", and I know it inside out. I intend never to sell it and will upgrade the engine at the earliest opportunity - using a 3rd party rather than Caterham.
Really there is no easy answer to this and if you are buying new or very nearly new and can afford it, I would sell you rcar and buy an R400.
Yellow SL #32