As Bill said, your current laptop might work fine for FCPX using transcoded footage, and there are various format options for transcoding (Apple Intermediate Codec AIC, ProRes 422, ProRes 422 HQ, ProRes 422 Proxy, ProRes 422 LT, etc.) that have various quality levels up to full professional quality. When you import your footage to your computer, make sure you keep your original AVCHD file structure, and don't modify them. That way, you keep all options open for the future. Transcoding will not modify your AVCHD files, but simply will create larger easier to edit copies of the files.
The advantage of transcoding is that it creates files that can be easily edited on not too powerful of a computer, or it makes it easy to edit many streams on a powerful computer without bogging the computer down. Also, if you are doing a feature film and doing color grading, etc., it makes sense to transcode all of the footage, as it will need to be done at some point.
The disadvantages of transcoding is that it takes time (I think that FCPX can do it in the background while you are editing) and it creates files much bigger than the original AVCHD files. However, hard drives are inexpensive these days - I recently bought 4 TB Seagate USB 3 drives at Costco for $180 each.
You have to realize that programs that edit AVCHD natively still have to do some transcoding during the edit process. For example, even cuts only editing need multiple frames to be transcoded at the edit points to generate new AVCHD versions. If that is the only editing you are doing, then it might make sense to edit "natively". But if you are doing many cuts, transitions, titles, color correction throughout, etc, then it begins to make more sense to transcode. If you are making feature length films, that sounds like the situation that you are in.
For more information about FCP and FCPX, see http://www.kenstone.net/fcp_homepage/fcp_homepage_index.html
Some years ago I used earlier versions of FCP, but for my modest needs now, I simply use iMovie. If I need to do more in the future, I'll purchase FCPX.