If you get the Lee holder (or Z-Pro for that matter), you are not locked in to using filters from those specific companies. But, first things first.
If you can't swing both types of filters, then definitely get the CPL first...its a very versatile filter and, in addition to its more popular uses, can also double as a 1-1/2 to 2-stop neutral density filter. You'll get much more use out of this filter initially.
For landscape shooting, I prefer using GND filters. However, if you get the CPL and have to forgo these for the time being, you can always shoot multiple exposures and blend them afterwards in Photoshop or other similar program. Just shoot one exposure for the foreground and one for the highlights...blend later.
A less expensive alternative than Lee or Singh-Ray filters are filters (GND's) from Hitech...very good filters at a moderate price. For the price of a single Lee filter, you can just about get two Hitechs (if considering S-R, you can get three Hitechs for the price of a single S-R filter). If you go this route, you'll want a holder that's versatile. Unfortunately, the Cokin P holder isn't...it has three fixed slots that cannot be removed. The two holders that are adjustable are the Lee foundation kit and the Cokin Z-Pro. I won't go into too much detail, but the Lee is the better holder of the two and is not much more expensive (last time I checked, it was about $12 more than the Z-Pro). If you search the forum, you'll come across a lot of discussions with regard to the two holders.
But definitely, if its one or the other, go with the CPL.