about 50g then . nice size , quite a few options really . arguably just big enough for a group of 5 or 6 discus , but deffinately in the bare minimum end of the range. why not try your hand at a planted tank?
maybe a breeding pair of something? or a biotope setup of some kind ?planted discus tank? some plants are dead easy to keep and dont need any maintenance
gravel is fine yep , as for what strains its all down to personal preference and of course what you can find . bogwood is good too 
/www.plantgeek.net/plantguide.php
/www.plantsalive.co.uk/index.php?_a=viewCat&catId=9
malawi mark
thing to consider with plants in a discus setup is wether or not they can handle the higher temps mate . the only 1 i had any joy with in my discus taank was hygrophilia polysperma. never even heard of half your list mate
well apart from java moss . and i killed that stone dead within 2 weeks 
id go ask in the planted section. malawi mark does have a nice ring to it i have to admit , but it implies i know a lot about malawis , when ive still got me african cichlid L plates on 