Check if it was corrosion proofed from new or at least before it was dismantled. If it hasn't been done then the chances are high that there will be a LOT of metal to replace. The lower fuselage and the lowest point of the wings (leading edges if they have been stood on them) are the most likely where 15 years worth of condensation has been gently turning the aluminium to expensive dust.
If the engine hasn't been properly inhibited then the chances are again high that corrosion will have set in The crankshaft and camshaft may be suffering expensive terminal corrosion which may also affect the internal surfaces of the crankcase.
I sympathise with your desire to see a straight tail C150 take to the air again (there used to be a lovely one in Haverfordwest) but I'd start negotiating at around £2-£3,000 and be prepared to end up selling the remaining good bits on eBay.
Spending some time and cash on an experienced engineer with a borescope, mirrors and torches should allow you to identify the majority of the corrosion and seized bearings etc which will help you assess the potential damage and repair cost.
Where were you thinking of doing the rebuild? I know of a decent hangar relatively near you where rebuilding work has been done before and there is a friendly engineer available.