Welcome snoopy,
Another 90's diesel joins the fold! I read your travails on the other forum, and was pleasantly entertained; I'm glad to hear it's all running well now.
I've ditched my veg-oil experiment, I ran mine 50/50 with whatever rapeseed oil the supermarket had during the summer months, but with the mileage I do (<2000 miles p.a.) it's simply not worth it. Hang on a sec, that's only 4 tanks of fuel a year... yeah that's about right. I experimented simply to see whether the old girl would take it, and yes, it did.
I'd be surprised if the veg was the root cause of your HG failing, more likely less attention to maintenance in the car's history, as you hinted. Silversaloon will likely be on here to welcome you aboard, and he's been on veg with his multivalve E300TD for 40k+ miles.
Running through with neat injector cleaner will definitely quieten-down the injectors, though at 100K miles, isn't it about time to change them anyway? I honestly don't know. I used Forte Diesel Treatment, and it also has a biocide to kill the algae that is no-doubt growing in your tank. Anyway, keep changing the pre-filter and main fuel filters every 2000 odd miles and enjoy your car!
About this leather interior, they do pop up from time to time on ebay, but finding one in a suitable colour might be a pain. Re-colouring leather isn't a huge chore if you can't find one in your colour. They are getting harder to source now, I was fortunate enough to find one two years ago for £117 on ebay, in excellent condition apart from wear on the driver's seat bolster.
Ebay.de is also well worth a gander, search for: "w124 leder" or "w124 leder sitze".