isc interspire shopping cart roll back to 6.1x stable version, and some revisions

I have a website running ISC 6.18 . as it turns out, this version turned out to be very unstable and full of bugs. now, all of a sudden, my website is only showing a blank white page, after having experienced other small problems in the past which were corrected... its a real pain to deal with these small problems each time, so we want to roll back to a stable version. I need an expert on ISC cart and php, mysql to revert my site back to something more stable, like 6.11, or 6.14, using the existing database. it is important that all customer and order information is retained, as well as my products, webpages etc. so it will most likely require getting the current site working first, then do the changes required. I need the template modified to add a some permanent logos and links to the right hand side column, and add an affiliate function, with an affiliate logon page for each affiliates see their performance/earnings etc. it is important that the right person can get on this right away, and resolve this white page issue and have my site up and running asap first, then work on the roll-back , changes to tempate, affliate addon section etc. with min. interruptions my budget for the above is $150. work may be on-going for the right individual with additional pay in the future, as we are looking for a reliable webmaster for our site to take care of any problems or revisions, and also do some SEO work through link building, site submissions, etc.