Welcome to TFF
For starters, tropical is freshwater

you mean coldwater I assume.
I think if you do some research, you'll be able to pick up a great second hand tank. If you are patient, you'll find great deals like this too -
My link
Some people even give away fish they can't keep... Its a bit awkward on a low budget but I wouldn't say it was impossible at all, it would require a bit of DIY for certain aspects of the tank, maybe plants would be a bit expensive on top of everything, but if you could find some slate (or other inert rocks) you could scape the tank nicely.
The link I showed you was more than your budget I know, but if you look around on the classified website on this site, there are plently of other sites too. Ebay as well...
If you're looking to buy something brand new you're going to struggle though!
Essential things, like liquid water testing kits, are going to be atleast 10 quid from ebay, mine was 20 from amazon. I think over time you would have to spend more than 50, and 30 is near impossible i'd say lol
but say, argos play sand, 3-4 quid as a substrate for your tank, try and find some slate or something to put in and put some cheap hardy plants (vallisneria spirallis or something). You've still got a heater, filter and lighting to buy.