The download speed, for most people, is the important factor. Download speeds are normally higher than upload speeds - this is because most people download alot of info - eg reading mails, downloading files, online games etc, but do not upload much info.
What do you mean by moderate usage? Will you just use it for email, browsing the web - if this is the case I would say 1Mb is enough. If you are downloading alot of files, or gaming online then 2Mb + would be recommended.
Note that your download speed can be affected by the contention ratio.
The upload/download limits per month mean that if you go over the limit set then the ISP will charge you extra. In most cases, especially if you are a moderate user, then you will not hit the download limit - I think that in most cases the limit is just a rough indication (this is what I was told when I signed up) - it's to prevent people downloading stuff 24x7.