well the little bit of rot i've seen in the photos you've linked to (i've looked at all 11 and will ignore the bulkhead for now) of the chassis doesn't appear to show anything that can't be overcome, we could do with cleaned areas to give clearer pictures
you probably have rot in the chassis rails where they join the rear crossmember which is easy enough to repair once the crossmember is dissected to remove it (don't just cut the chassis rails to get it off), i cut my crossmember to bits to remove it and then carefully cut the welds to get the remaining little bits off (i made my jig up first) just don't rush, look and think twice before attacking it with the grinder, i cut mine off using 1mm cutting discs as they give the tidiest most accurate cuts
do not buy a generic crossmember with extensions as they are designed for the 88" chassis, one without the extensions will allow you to repair your chassis rails and fit it perfectly (i've seen someone struggling to fit one with the extensions onto a 109)
both outriggers can be bought as can the rear suspension hanger, generic bulkhead outriggers are for the 88" chassis but can still be used on the 109
i spent nine months rebuilding my 109 (not every spare moment but an awful lot, i also didn't completely strip mine in one go) take on one repair at a time, don't cut everything off just do one job at a time, you can then see progress as you remove, replace and paint each part when you do it one bit at a time plus it gives you reference points for measurements
you're also lucky as you're starting this project as the daylight hours are getting longer and the weather is about to warm up, i started mine in November 2010 as we were running out of daylight and had three or four inches of ice on my pond in December (i had a 200gallon fishtank freeze solid from top to bottom
biggest icecube you've ever seen :bump2: )
find yourself a local fabricator and buy/scrounge some offcut 2mm sheet steel so you can cut your own repair plates to let into the chassis where needed, carefully scribed you can cut it with 1mm cutting discs to the shape/size you need it
G clamps and fabricators magnets are very useful items as are molegrips to assist with holding parts in place while you tack them, tack only before welding to ensure you've got it in the correct position, most important to check before finally welding on permanently