The kit you are referring to includes Prime, Stability & Clarity.
I can?t really comment on Stability & Clarity as I?ve never used them and as much as I respect Seachem and their products I tend to be suspicious of products that claim to ?fast track? and seed a biofilter. The only way the biofilter can develop is through a gradual process of introducing ammonia into a tank preferably by fish-less cycling - that could be what this Stability contains, ammonia but as your fish are already producing ammonia for the tank I would be unsure as to its benefits.
Clarity reduces cloudiness in tanks but I don?t know of any other benefits to it?s use.
I would just get a big bottle of Prime rather than get the headstart kit but it?s only my personal opinion ? I am happy to be shot down by anyone else who has a better experience of these other 2 products.
I am also quite ?anti-chemical? when it comes to my tank but again of course, this is just my approach.
Some fishkeepers like pumping their tanks full of all sorts of products that claim to reduce algae / reduce cloudiness / de-stress fish / raise or lower ph etc etc but in reality you shouldn?t need any more than a good de-chlorinator as a chemical to add to your tank, if you find yourself using lots of chemicals for things you could be disguising an underlying problem with the tank that needs addressing.
Don't know what you're budget is but perhaps in the short term consider a Really Useful Box, these are great for temporary accomodation. A 145l version is available for around ?40-50 depending on where you go (I've found Ryman is usually cheapest for these). While still a little on the small side for what you've got it would be miles better than what you've got. You will also need a suitably rated filter, which you should be able to get fairly cheaply if you look around.
That's a really good shout from james15, ok it's not an "aquarium" but it would be a better option for the fish and would mean they are much more likely to thrive and less likely to suffer the ill effects of ammonia / nitrite poisoning.
Certainly a good short-term solution until you get a bigger tank - and you can keep the box for any other fish-tank related uses in the future.