It doesn’t need to be a long, flowing letter. The right words, no matter how many or few there are of them, mean enough.
As far as during the engagement is concerned, though you really have to be careful how intimate the letters are – it’s really tough when you have stuff you really want to say but not sure how to when you’re not allowed to use certain wording. That’s why I found my engagement really hard, cause I couldn’t express myself to my husband, and he thought I was being quiet and non-responsive.
You don’t need to buy him a gift for the yichud room though. A chassan is supposed to give his kalla gifts, as we see in tanach, but there is no mention of the other way around.
Mazal tov!