I don't think I've seen one of these that hasn't got some sort of leak from somewhere in the boiler. You may need to order the O rings from merchant as they may be an odd size.
If you are on about a leak from the secondary heat exchanged to the right of the fan then you will need new O rings in my opinion. You can try and clean them up and grease with silicon grease but I wouldn't risk it for a couple of quid on a new O ring set!
U normally use a half inch female iron (brass) to connect into those threads. Looks like a nut and olive has been used. Works but not the best way to do it. Although Flexi hoses are used quite often.
To be honest it don't look that bad. Stop cock is a little low but seen worse. I would lag the mdpe to protect it then fill the hole with sand and cement. Fit a new waste adapter to make sure the seal is ok! Job done.
I would go for a Worcester 28cdi compact. It's still 24 kW on heating but with better hot water delivery and seeing as you have a mixer shower you would see better performance as long your incoming mains water supply is adequate.
I would recommend that you get the tank cleaned every twelve months and disinfected with chlorine dioxide spray. Then I wouldn't worry about drinking it! I worked on old peoples homes and this was pretty common.
You need to cut it! As above the trap is too deep. Cut it at horizontal section and remake with shallow trap. At least you can inspect horizontal section when you cut out trap. Easy job even for DIYer.
At the end of the day the filter will still collect craap. Yes the ideal place is on the return. It can some times be impracticable do this so it goes wherever is possible. Plenty of boilers out there with no filter or power flush. Theses jobs are tight for many installers so be grateful you...