Well first, welcome to the forum! Sounds you got the overlanding bug early. Although I recall some cross Canada trips in a 1969 Mustang in my youth. But this is a good place to get some info.
As far as Africa goes...the cheapest option is the west coast as you can take a ferry over to France, drive down thru Spain and ferry into Morocco. From there Africa lies before you. As far as costs go, it depends on a lot of things. How fast you plan to travel, what countries you want to hit and what your plan for getting home is. Most people work out a daily expense rate, something like:
Average distance travelled per day: 100 miles (£15-£20 at 30 mpg depending on local fuel price and terrain)
Food per day: £5 (buy local fresh)
Camping: £10 (if you camp in pay camp sites...sometimes much cheaper depending on country)
Vehicle Misc: £2 per day (oils, bulbs, bits and bobs)
So, based on the above you would have to budget £32/day. Now some days you might not travel, or free camp somewhere so your expense are down...but other days you may cover a couple hundred miles and have to have something fixed on the truck. So a rough budget is £950 per month and a reserve of maybe £1000 in case of emergency. You would also have to do some research and see what the costs for visas are at the countries you want to cross.
There is an American in a Jeep doing the route you are considering now. He has a blog where he discusses all the issues he is facing. The link is:http://theroadchoseme.com/
My biggest advice if you are hitting the shows is to ignore all the traders selling flashy bits who tell you that 'you absolutely HAVE to have this expensive bit of kit to overland!.' Find the people who have actually done the trips and you will find out what you actually need. But keeping the weight down is important for reducing strain on your vehicle's suspension and drive train as well as on your fuel mileage.
Look forward to hearing more about your vehicles!
"We've got a date with Destiny...and she's ordered the lobster!"