Lissandra isn't fun imo or very good. Also, I'd never spend RP on a champion, unless you desperately want them.
If I were you i'd hold onto it until something comes out that you think "Yes, thats definitely worth $20", then go ahead and buy that. If you think Liss is worth $10, then go ahead and buy her. If you dont/can't add RP often, keep in mind that Worlds and Harrowing are both coming up soon meaning new summoner icons, skins and ward skins (potentially) for both events.
EDIT: Just realized that you do actually want to spend some on a champion for top lane. Id recommend checking out the august sales page and seeing if any champions are in there that you like and wouldn't mind waiting up to a couple weeks for.
Fun top laners are champions like Riven, Gnar, Maokai, Lulu*, Nidalee, Darius, Akali, J4 , Cass etc.