TH1,
I would strongly advise you to get an engineer, or local builder that you can trust (easier said than done I know) involved before you tried to tackle this on your own, unless you or someone very close to you has plenty of building experience.
One of the advantages of a local builder is that he/she would know what trades to bring in and when (e.g. general labourer, carpenter, plasterer, painter etc.). I would suggest that you make contact with one or two asked them for a quote and some references and see where that takes you before you start it on your own. This process in itself will tell you a lot.
A worst case scenario would be that you start the work, then end up having to call in a builder half way through. If the problems are worse than you think it would definitely be worthwhile getting an engineer's opinion.
In addition to that, if you don't insulate properly, you may end up leaving variuos cold bridges etc. which would result in teh additional insulation etc. being totally ineffective, and a wasted investment. You also could end up making the condensation worse.
Don't mean to alarm you but...you know!
J2K