di2 is not worth it one bit and if you take a tumble and smash the rear mech area I'm told that little box at the back is £600 to replace, could be less could be more. What problems do you have with cable gears, once dialed in which is pretty easy they generally stay that way. And I suspect you know this.
Personally I am paying for 2 bikes on 0% at the moment (1 finishes in 2 months) and I still have the £3600 they cost in the bank. If your confident you can keep the payments up then I'd go that way.
Having said that I have 2 others which I paid outright for the more expensive of which is on a )% credit card and the direct debit set to pay it of in the 39 month 0% offer period.
Your call, and personally wouldn't go over £200-£2500 max on a bike.