I don't know anything about the Belkin and not going looking at this time of night, but there should be no problem running both a Dell and a Mac wirelessly from the O2 box, though it doesn't seem to be the best for wireless on the market.
However I think there is something wrong in your current setup if one machine loses wireless connection when the other goes online! Losing wireless in different parts of the house is quite common though

.
It sounds as though you have checked you can get O2 LLU. Or haven't you? If you can, then going for O2 Premium at £9.79pm would be wiser than O2 Standard at the moment given the fact that there is a sale on and on Premium and Pro the first three months are free.
Also note that if you sign up completely online via Quidco, having cleared your cookies first, (or using your daughter's machine if it hasn't ever accessed the O2 web site, though you probably can't be sure of that), then you should get a £70 cash-back (less £5 if it is your first Quidco deal in a year).
I'm not an expert on multiple router setups but there are many on these forums. I think it is possible that once things stabilise in the first few days on O2 that you may be able to link the two routers by cable possibly just in different areas of the room, disabling the modem and DHCP on one of them, and get better radio coverage that way. Or link them in different parts of the house using Homeplugs which would connect them through the mains. Food for thought!
If that idea appeals then the place to ask about that would be in the
Home Networking forum.
I agree your stats probably aren't important given your download speed, but you never know. Down and up connection speed/data rate, attenuation and noise margin (SNRM) are the most important. There may be room for improvement there.