I bought mine already converted and it's always run fine, no noticable difference in performance compared to petrol even up to the red line. Only a little more expensive to buy compared with a petrol version of similar age/spec so it works out hugely cheaper than doing the conversion myself.
I have the full size tank in the boot, suits me fine as it gives a range over 400 miles which a spare wheel tank won't get anywhere near. I still have my spare wheel too, it leaves a reasonable amount of space in the boot but I appreciate this won't suit everyone.
If you go for the spare wheel tank, the options are either carry a full size spare in the boot, get a suitable space-saver wheel, or carry tyre weld. I think the latter is OK as a get-me-home type repair as long as it's a minor puncture. More serious tyre damage won't be repairable. Personally I'd say it should be OK, just make sure you've got breakdown cover and ensure they will help out for punctures as some of them will charge if you don't have a spare wheel.