As has been said before paper maps don’t give width and weight limits…and for bridge heights i think only the truckers atlas provides these… 
wideload:
I am thinking of buying a tom tom sat nav kit. if it runs on 12 volt whats the best dropper to use.
why not try this free GPS HERE
for help with low bridges on sat nav try www.lowbridges.com
Taskman:
But if you have any intentions that a SatNav kit of any price will replace your locker full of street guides it won’t take it from me, they are geared up for the motor car and any vehicle that does not come within any local weight and width limits
As are most maps, are they not? SatNav has replaced every single map I used to carry, and given the area I cover that was a lot of maps - they filled a wheely bin.
Taskman:
because this bit of kit will send you down them.
can I just point out it isn’t the law that you have to follow the directions the SatNav gives you, you can ignore the instruction to use a road if signs or instinct tells you it isn’t a good idea. 
Pretty much the same applies to a map when you have planned a route and find your chosen route cannot be followed because of a weight restriction or otherwise. The difference is that with SatNav it will quickly re-route you without you have to do anything, other than concentrate on driving, whereas a map will require you to look at it and start working out a way round the restriction.
re lowbridges.com dont waste your money there are free databases around that just as acurate 10 minutes on google or the site in my signiture