Hi,
I have the exact same tank as you and I have just puchased and set up today an eheim pro 3 2075 external filter.
My reasoning for this is that the filter will easily cope with the bio load in the tank and when it has finished seeding I am planning on removing all the internal filter, heater and housing to create a bit more space in the tank.
Eheims are regarded as extremely good external filters but they do come at a price, mine was £170 from charterhouse-aquatics.co.uk
There are plenty of other external filters that would do the same job for you. whichever you go for you would need to run it along side your internal filter for 6-8 weeks for the good bacteria to build up in the new external, this is known as 'seeding' a filter.
Andy