To give some idea of the baseline cost, I built my 1.6 SDV absolutely stock from the factory, using the Westfield starter and completion kit. Obviously, you pay extra for the finishing stuff in the completion kit, but it's all IVA compliant and you know it fits. With a donor that cost £600, the actual cost to get mine on the road (including IVA, Registration, Tax, tools etc) was just over £11k. Therefore, you will need to save £5k from a bog standard build to meet your budget.
I'm with Gary, £6k is just too optimistic for the average builder and I guess £6k is theoretically possible for those on a higher plane, like Smokey Mow but even he would struggle. You might get away with £7.5/£8K with some serious scrabbling and compromise. Personally, I'd wait until the budget was healthier or buy one already built and tweak it.