The Mines of Moria expansion would be a good one to get now because it includes the Warden and Rune-keeper classes plus 2 extra character slots. Other than that, you can probably wait until you're closer to level for the other expansions.
Leveling group on Brandywine W,Su 8:30-10:30pm EST (current level: 53)
All three Rohirrim Style Mounts with a blanket & emblem in the colours of green, red and white
All three sets of matching cosmetic Rohirrim Style armour in the colours of green, red and white
An in-game title - Guard of the Isen
Access to the Path of the Fellowship Quest Packs including Trollshaws, Eregion, Lothlórien and Moria plus the Mirkwood Quest Pack as a special bonus
1,000 Turbine Points for use in the LOTRO Store
The price for that package is steep however, 49.99$ but considering the new pre-purcase for Riders of Rohan which weighs in at 69.99$ for the legendary edition, and doesn't even include any Turbine Points; the Isengard package is a good deal.
The other option you could go for, is to wait for Moria to get on sale in the LOTRO online store, but that rarely happens, the same goes for Mirkwood.