Welcome to 40KOnline erik007. Enjoy your stay here.
In my estimation Black Templars are a slightly more complicated army than normal Space marines, but only slightly so. It's mostly a matter of getting into assault as fast as possible while taking as little fire as possible. They're still Space Marine infantry at heart, so don't require huge thought. That should help in winning games.
Conversions are easy thanks to the highly modular Space Marine plastic range. You should also be able to pick up some models for relatively little money secondhand on the internet. The market is flooded with them. Many people start (and quit) Space Marine armies.
As painting goes, they're exceedingly fast and easy to paint as well if you know what you're doing. The main problem is posed by the shoulderpads. You'll want to prime the models black and the shoulderpads white separately, then stick them on after most of the painting is done. Getting a good even white over a black undercoat is exceedingly difficult. The black armour is very easily painted with codex or shadow grey and the edging technique. That's 80% of the model done already in little more than 2 priming and one painting stage. Details like bare metal purity seals and the like can then be done to complete the model in no time. The stark black/white contrast makes Black Templar armies look very effective on the table. It helps to use a contrasting base (and details on the model) to add some more colour.