Time to Change, if you are happy to appoint your best friends and they're happy to accept the appointment, then it will be very straightforward to include that in your will.
I'm a lawyer with over 20 yrs experience in High Street practice and now run my own Will Writing firm.
I'd suggest that the Guardianship appointment covers your younger three, as obviously your 16 year old is still a minor, but you could then go on to express the wish that they consult with your 19 year old (and, from their 18th birthday, the 16 yr old) on major decisions regarding the younger two's care.
I'd also suggest that you have a clause in the Will allowing the Guardians access to the trust fund to cover the children's expenses, but that would have to be with the Trustees' consent. The Trustees are the people whom you would appoint as Executors, who normally also take on the job of looking after the trust fund if any of the children are under 18 or 21 at your death.
Obviously the Execs/Trustees should be entirely separate and independent from the Guardians.
Cost wise, I charge £85 for a single, £140 for a couple. I have a paid for advert on the Small Business section of Classifieds ("5* Will Writing Service recommended by Mumsnetters") if you're interested.
You will pay more at a High Street solicitors'. I still practice part time in a High Street firm and charge £205 for a single, £288 for a couple in that practice.