Err, did you not see my reply to your last thread/question?- its a piece of cake to convert the lightning to electric. You've asked 'what should I do?' about 3 times now- its your money; you have to decide how you want to spend it, based on:
1. price
2. parts availability
3. ease of conversion
4. compatibility with your existing electronics/ cost of new electronics that will fit/work.
5. smile factor- if it doesnt do anything for you in terms of looks, chances are you wont enjoy using it so much.
6. Feedback from existing users- you have to do alot more reading to see what people think about certain models and conversion parts/routes etc. That goes for performance and handling too.
If your lipos wont fit, you may have to accept that you will need to buy new ones, or modify the battery tray/ make/buy new ones that are large enough.
If you cant find parts for the model you like in the Uk, then either bite the bullet and accept you have to order from abroad (we live in Blighty, 99% of everything has to come from abroad that I buy pretty much- I learnt to deal with that a long time ago) or choose a different model that has better parts support. I've had to spend days searching and researching to see if parts are availbale for certain builds I've wanted to do, but ended up not going through with it as parts are too scare at home & abroad even.
An out of the box 'e' model of any vehicle will force you to spend alot more initially, but since its new it will have easy to find spares and support- if not in the Uk, then from the states, but again, you learn to deal with it. A conversion of a nitro roller will be tons cheaper and let you decide on certain feature like esc and rx placement, and battery tray size & location(s). Zillions of conversions of things that are similar to your vehicle choices so far- the 8t is a very common conversion and spares are easy to find- look at more existing builds and conversion and see what people are saying about how good they are to drive, or not, and how durable ( or not ) the parts are.
Seeing as you just want to go fast onroad though, you could buy a 1/10 TC, stretch the chassis and make it go 100mph without much trouble, ot do like Joe Ford did and stick a MMM&2650kv motor on 6s in his 1/8 buggy (look that up, I cant remember what he used now, but it was very recent)- easily 80mph down his short street, with more room he could be pushing 90-100mph or more, just with one motor mount and a battery tray needing to be installed to complete the conversion.
Happy researching