Customer needs an LTM+DNS license. Would we recommend R2600 or R2800?

DNS module and DNS traffic itself is quite negligible, even for VE
https://my.f5.com/manage/s/article/K13114858

and moreover client also will also caches dns response (for duration defined in f5 dns config).

so basically you only need to consider load of LTM.
you can use basic calculation from here:
F5 BIG-IP Sizing Requirement | DevCentral