As Dave says, sumps are very good at dissipating carbon dioxide, and adding oxygen, but that is the opposit of what you want in a planted tank. You best bet is a canister
Tetratec have a EX2400, that would give 4 times turn over. Double two up to give a redundant system and the flow required for an EI tank, but that would set you back £400 near enough for a newly released filter that may be a risk...
The Fluval FX5 would cut it well, but is unproven as yet for long-term reliability, hence is also a risk, especially when you consider Fluvals track record with exturnals.....
Eheim are a proven brand and highly recomended on here, but IME their reliability isn't bullet proof. IMO the 2028 wouldn't be enough. I'd consider a 2080 a better option, particularly doubled upi with a 2028 for flow
Rena XP4's give more torque than the motor can handle, with many users reporting that the impeller shaft and/or the impeller itself snapping on starting after it's been cleaned a few times..... I'd avoid this one...
This is a tough desision to make, mainly on personal preferance. I'd give the Tetratecs a miss here, considering that this is a new model and may turn out like the Rena XP4...
The Fluval FX5 is the cheapest option, but hasn't yet been proven. The 2028 IMO won't be enough. The XP4's are supposidly poor reliability wise, though I haven't used this particular model, the Tetratec would work but are expencive and none-proven yet and my Eheim spec is also going to set you back a bit... If it were me, I'd go with a Eheim 2080 and a 2028 doubled up, if it wasen't for my past experience with them... With my past experience, I'd probibly go with an FX5 Fluval, but the choice is yours
HTH
Rabbut