I think you should wait a while, because you really don't seem to know a lot about millipedes even though you went ahead and bought one. You've also made a post saying that you're worried about the health of your millipede and already you're thinking about getting something else! I would just see how you get on with the millipede first and then decide if you want something else.
I'm not trying to sound mean or anything, I'm just stating my opinion and saying what I see. If you can't look after a millipede (which is about the simplest thing a person CAN look after) I don't think you should be thinking about reptiles or amphibians as they require much more specialist care and need a lot of research before decisions are made. Asking a couple of vague questions on a forum that isn't even about reptiles or amphibians and having a quick look on Google isn't the sort of research I'm talking about. Buy some books, ask questions on species-specific forums and find as many caresheets as you can by Googling. Also the tank should always be setup at least a week in advance of getting the animal to make sure that all the equipment is functioning correctly. Also please bear in mind that most reptiles will live in excess of 15 years (with there being records of leopard geckos having lived 30 years or more).
That said, a leopard gecko is about the easiest reptile to begin with and I do recommend them because they are hardy, interesting, pretty and have lots of character
. In the next few weeks, just see how you get on with the millipede and keep doing lots of research on any other animals you are interested in. You really can never research too much! A website with really excellent caresheets about leopard geckos is this one:
http/www.thegeckospot.net
I also highly recommend The Leopard Gecko Manual by Phillipe de Vosjoli, Roger J. Klingenberg, Ron Tremper and Brian Viets. It's really cheap at around £5 from Amazon. Good luck!