Interesting one. I was reading your new build thread and thought the same, golf r plus 2.5 = rs3 money, but did not want to spam it.
If it were me I'd keep the polo and then get a r300 superlight. No matter how much you spend on a golf or rs3, it will not come close to the thrills the caterham would give you.
Modding, in my own experience, does not give good value for money, you would be better off spending it on petrol and track days. Enjoying the car rather than chasing numbers on a dyno.