I'm not familiar with the aquastyle 510 but there are basics for this kind of thing with any tank.
Move all the ceramic media into the filter of the Aquastyle tank - unlike the biorbe there is not an undergravel filter so alll the media has to be in the filter so that water flows over it. If the ceramics looks to be at risk of falling through the container or escaping in some way you can get filter bags to contain it. The filter media is where 99.9% of the friendly bacteria that clean up the water live. The biorb also has chemical filtration sponges, in theory these are not required but if you can fit them in then move them over too - cutting the sponges may be required and this is ok.
The water should be transferred because it's got the right chemistry for the fish - it's what they're used to so you'll get less problems if you move it. Bag the fish like they do at the fish store or put about 8L of tank water into a bucket and put the fish in that - cover the bucket with a towel to stop them jumping out or getting stressed.
When the water is moved, top up with dechlorinated tap water and acclimatise the fish as if they where new purchases.
The blood vessels can be a symptom of several problems including high nitrate or nitrite, poisoning, suffocation and/or septicaemia (blood poisoning/systemic bacterial infection). At this stage water changes are the right thing to do but I'd be very careful about big changes like that. It can cause thermal or osmotic shock.
Test the water kit on your tap water. You can compare this to the printed reports from your water company. They're normally on a website but if not then you can call them for the details. If it's not close then something is wrong - and it's more likely to be your kit that theirs. It's probably worth taking a water sample from the pond to the lfs and paying them the 50p or whatever they charge to test ammonia, nitrite, nitrate and ph for you.
Always buy a filter that's bigger than you need Media capacity is generally more important than lph Buy eheim and you won't regret it.
and in direct answer to your questions: 1)Is there any i should avoid just ebay hong-kong imports and unknown brands.
2)My tank is 106ltrs, if i got a larger filter for the tank, would this cause a more flow in my tank ? Generally yes, a little.
3)Can you get any with adjustable flow ? Most either come with a tap or you can buy one as an extra. The tap goes on the filter outlet to restrict flow to whatever you want. Generally though you can get by using a spray bar or pointing the outlet against the back wall or surface to spread it.
4)We are not talking silly money ! More money does not always buy you a better filter. The eheim classics are cheap and effective.