to allow you to turn items up to the 40" capacity you are going to need a full size lath and possibly a Bed Extension depending on your choice. Have a look at the Jet series of laths but there are many others you should really consider before buying. Choose the options some of these manufacturers offer very carefully and ensure that the lath you buy can allow your skills to grow and tackle some of the more complex turnings as you progress.
This is a hard one to answer given the choices out there. A lot of it depends on what you can see yourself turning initially and what you may progress toward such as out-board burls and very large bowls. Price range is always a factor at some point but digital readouts, inboard-outboard head swivel and revers are very nice to have. Just keep an eye on the availability of attachments that manufacturers have out there and which machines they are capable of being used on. Things such as this may help you out on your purchase but I am sure there are other issues I have not touched on here.
Let us know what you decide on.