I would imagine that as long as it's lead free, you could use any gloss trim for internal structures. I recently used Taubman's water-based gloss for all my metal door frames and MDF/wooden skirting boards, it was pretty easy to apply compared to oil-based enamels. And much easier to clean up, too. Make sure you prep properly, ie, scratching up aany varnish, and applying a coat of sealant/3-in-one undercoats first. You'll get a better finish on it with gloss over sealant, but expect to do at least 2 coats of gloss still.

Great idea re the craft letters, but I don't know how best to fix them to the frame. Glue would be quickest (Selleys no more nails, or something) but i imagine that glue and screws would be much more secure. but then you'd need to buy more products - the glue, whatever drill bit you need to drill through the letters and have the little recess to let the screw head sit flat, and then a filler of some sort to cover up the screw head. Try going into Bunnings and asking advice in the appropriate section. If the attendant starts reading labels, rather than knowing of the top of their head, ask nicely for a staff memeber who's more familiar with these products to advise you instead.

Have fun.