Hi Ellena,
Welcome to the forum.
I'm just going to put my two pennyworth in here for a second. When you choose a filter its worth remembering that if you choose a slightly larger filter than you need the flow can be can be turned down to suit (on most filters nowdays) and turned back up again to compensate for losses of flow should they arrise (blocked media, extreme plant growth etc.). The larger capacity filter will more often than not take a larger volume of media which will help with your filtering and if you decide to upgrade your tank to somthing larger in the future could still be usefull. Obviously you don't want complete overkill but if you go for somthing like a tetratech EX 600 and turn the flow down it won't cost a great deal more than its smaller cousin the EX 400 but would be more addaptable, providing you have the room.
Also bear in mind that you will no doubt loose about half the filters output rating as soon as you fill it up and place it underneath the tank. This is because the manufacturers tend to rate the filters with no media in them and a zero head height (ie. no water to pump uphill). This has been taken into account though when you apply the 10x general rule of thumb for flow ratings on planted tanks (total flow should be rated at 10x tank volume), your 27Litre tank will require a filter with a rated output of at least 270Litres an hour or more.
Therefore the hydor prime 10 would be ok, somthing like an Ex 400 would give you quite a bit of room for manouvre and its cousin the EX 600 turned down would have a huge filter media capacity for a tank of that size and may be more benificial to you in the long run.
Which filter to buy is very much a personal choice, but I can recomend the tetratech range of external filters as I use these myself and they seem to be offering the best overall value for money at the moment. Shop around though as prices vary considerably. Warehouse aquatics, zooplus or pets products online seem to always have these at reasonable prices to name a few and there are usually second hand deals to be had on ebay. If you can afford it Eheim have a great reputation and are very well made and there is probably one in there classic range that would suit your requirements. Personally I would steer clear of the eco range though, just heard some duff reports about them thats all, didn't mean to offend anyone.
Whatever filter you decide to go with its worth remembering that it will only be as good as the media that you use in it. Sometimes filters are priced without media and sometimes with a full range of top quality media. Some filters may be cheap initially but require filter media that you have to keep buying from the specific manufacturers which are invarioubly expensive.
The external heater will be fine. Its not like there is a lot of choice anyway if youv'e decided that external is the way you want to go. You could use a more suitable 50W or so standard heater and spend the money you save on a better filter, or beer!!!
Regards, Chris.