I have an 8.4m Avon with a 340HP Cat Diesel and a Hamilton Jet...
It is a great boat.. however.. if I could go back 3 years... I would have bout a similar sized boat with two outboard engines... Anything goes wrong on a jet and it is a major effort to fix it.. Outboards... you just take off the boat with 4 bolts...
Fuel economy.. we were getting 3 litres to every mile.. at £0.45 a litre sounds good.. but then if RED diesel goes that gets exspensive.
Also when I bought it for a commercial operation I then found out after that you can claim VAT back on petrol.. so that would make outboards a realistic option..
A jet drive is a lot of fun, good turns etc.. but twin outboards do the same.. are lighter and as you say easier to change parts and impellers etc.
Avon do the same boat we have new in outboard and jet drive mode..
Jets in shallower water are good, but do suck up things, Twin outboards with powertrim can do shallow..
Having driven the MOD pacific 22 with jet drives on the ships I work on... they are fun too.. but suffered a lot when new with problems, and they seem to go wrong far more than the sterndrive versions..
Have some photos of ours during 'rebuild' if you want to see...