Sure you can go with a hard tail 29er - and will notice immediate improvements in your riding. The lefty is not a gimmick - pretty cool design, and more importantly works great on trails (tracks very nicely and feels "more solid").
For the money, you could opt for a Full suspension 29er.
Examples:
Cannondale Scapel 29er 4 - alu frame
Scott Spark 940 - alu frame
You'll find better components on the scott, and far less issues with a regular fork.
You are also less likely to have crackling sounds coming from the rear suspension.
Bottom line: The ride of a full suspension 29er is pretty hard to match. It's a more comfortable ride, so you can bike even more (which is the point of it all).
Both bikes: tried and extensively tested - they both climb like goats (even though they will weight about 2 pounds more than a hard tail.)