karvelis,
My head started spinning reading all the responses. If I understand correctly, you are trying to set up a layout for your grandson, have a limited budget, trying to decide which track to use 027 or regular (deluxe) O tubular track.
Forget the 027 - Since you have a 14x14 area the regular O tubular is a better option. Keep the 027 on hand for now if you cannot return it and that way you have something to compare with if you are out purchasing any used O Tubular track.
Regular O tubular track can be found used and good pricing. If it has manageable rust, there are two products out there that you can soak, rinse and dry off the track and be good to go. Metal Rescue and Evaporust. I use Metal Rescue and I can put my hands in it without gloves. It can be poured down the drains or out in the yard - it is a green product. (I don't have O Tubular track, but use Metal Rescue to get rust of other metal train parts)
Forget about all the other tracks mentioned, get some more O Tubular and get those trains running for you and your grandson. Get a couple of quarts of latex flat paint in Green, Brown and Grey and you can paint, grass, dirt and streets on the plywood before you screw the track down. Think about building a basic tunnel - kids love tunnels.
Keep a piece of 027 with you when you are out at garage sales so you can spot the difference. You can get good deals on used O on eBay if you are careful. Also on 14x14 try and set up for three independent loops. I have kids and the more action you have going on the layout the more fun they will have. Consider Lionel operating Accessories, like the coal loader, log loader, gantry crane, dump cars and gondolas. Fancy scenery is not going to hold a young boy's interest, but pushing buttons and flipping switches will.
Lots of great stuff here on OGR - Be fore warned the collective IQ here can muck things up with all the smart guys, but everyone means well and wants to help.