Looks like a cool rig!
Would it not be better to spend a good amount on a massive service and maybe install lpg to your disco?
If the mpg is all that your disheartened with then that would go someway to alleviating that problem (I know lpg has gone up recently but still cheaper than unleaded).
At least you know the disco, and you could do axle/swivel/hub rebuilds, install the lpg at a fraction of what it would cost to buy a series and rebuild it and then still have an uneconomical, underpowered vehicle!?
But if you want a series that's a good enough reason.
I bought my series 3 when I was 18. I drove it for about 6 months and the lights stopped working, so I did the sensible thing and stripped it to the ground, where various bits are scattered around my dads yard. I had huge plans to put it on my rrc chassis etc and it's only now, after having been married, had a baby, bought a house and rebuild my own 90 onto a galvy chassis that, now at 34, want to put the series back together as standard. They are great fun, and full of character. It's the journey you have to enjoy with them, not necessarily the destination!
Best of luck whichever you choose!
