Also, how can I be certain the t410 supports the SSD?
First things first - compatability. Check the specs for your exact model support.lenovo.com T410. The HDD tray may already have an adaptor, or it could be a SATA connection. Confusing by the spec sheet (attached)
Connector type: As noted for HDD option from link above.
My first impression is that the drive may be mSata type, not SATA.
So you may want to pull the drive first to ensure what you want to buy will connect.
This adaptor is just to demonstrat the difference in size of each connector type.
mSATA Size (I could not find a mSATA male plug for best compare, but you get an idea of size)
Is there any point in getting a 6gb SATAIII SSD if the laptop itself only has SATA I?
Only if the SATA III is econimical, if a similar sized SATA II is available for less money, that would be the one to go to. If your laptop has SATA I you will not get the SSD drive's advertised performanceat SATA II or III speeds. That being said, you should still see an improvment over the plater drive.
A clean install and using easy transfer would be a good option to ensure windows applies the correct options for the SSD (Trim enabled, prefetch/superfetch and indexing) are correctly set up. You can do this mannualy, but a clean install is a good option in my opinion.
As the capasity of drives differ, it may not be possible to do a true image transfer.