Nice to see some 58 Pattern webbing being used by a noob - vive la diversity!
If your walking boots are already worn in and comfortable, yeah go ahead, they should be ok. As has been said though, assault boots / tactical boots are intended to provide more ankle support in the kind of situations that energetic airsofters will get into. My favourites are
German Para Boots, Grade 1 surplus (so they are worn in but not so much that your feet have to conform to the shape of somebody else's) / at the moment you should expect to pay about £40 for a pair, certainly no more than £50 posted. You do not need to buy a size larger to allow for thick socks, they are already slightly bigger than shoes/trainers of the same size. Of course they are sized in Euro measurements so you can get a slightly better fit than boots made to British sizes. On the issue of authenticity in your loadout, serving British soldiers in the 80's often bought surplus German Para Boots after they had passed out because they are more comfortable than British Assault Boots - the MK5 GPB is obviously newer and has design improvements over those available in the 80's, but the external look is very similar. As has also been said, you are best off trying boots on before you buy, but if you measure your feet with a ruler/tape measure and look up a size conversion chart online, you'll probably be ok.
Shooting glasses are not really suitable for airsoft as the gaps around the edges can allow BB's to ricochet off your gun/equipment and/or stuff around you, which then approach your eyes from below/slightly behind you, pass through the gap, bounce off the interior of the lens and hit you in the eye - this is not speculation, IT HAS HAPPENED. If you find that your full face mask is not comfortable, or the inability to get things like drinks/sweets/gum into your mouth without disturbing your eyepro becomes too annoying, you should always make sure that any goggles you choose seal against your face all around your eyes. I would also recommend separate goggles and lower face mesh mask to protect your teeth. As far as I am aware no sites require under 18's to wear a full face mask, rather they require u18's to have their entire face protected, so a separate lower mask under goggles is fine. Bear in mind that you can always modify any lower face mask, bend it, fold it, cut parts off it, and/or add pads to the inside to make them more comfortable and to make them work with a particular set of goggles. For eg this is what i do to mine:
...the bit of velcro loops in the middle prevents the mesh rubbing the tip of my large-ish nose.