WiFi Recommendations

Just moved into a new apartment with my wife - actually the first time I've ever lived in a proper apartment block. So now I'm trying to deal with the WiFi issue...

The bad news first. The 2.4Ghz band is completely saturated - there must be >30 networks dotted across that band, so we experience frequent drop-outs and generally crappy behavior.

The good news. The 5Ghz band is empty! It's awesome! Great reception throughout the apartment, and network behaves like Kenneth Parcell.

So the solution seems obvious, but we've got that horrible problem where several devices won't play with 5Ghz (Nexus 7, Wii, Smart TV...). To get around this, I've set up two networks - one primary 5GHz network on the ISP supplied router, and then a secondary 2.4GHz network from an old Belkin router I should have chucked. This router is very old and tired, so I'm looking for recommendations for improving the whole situation.

Budget is negotiable, so where should I put it?

Bonus points for prioritizing my PC, which until now has always been hard-wired... but that's now not an option. It's currently using an awful USB WiFi adapter that must be destroyed soon...