Parabolic dishes are really expensive. The most inexpensive I have seen of any quality whatsoever was around $1,100; professional parabolic dishes start at around $2.5k and go up - and I mean UP! And that doesn't include the mic! If your lavs are or can be hard-wired and are of good quality they may be an option as plant mics if they have enough "drive" for a long cable run. Long shotguns offer better off-axis rejection than short shotguns, but you will still get quite a bit of ambient sound and they are difficult to use even under the best circumstances.; they are also relatively expensive.
The Zoom is okay, but for delicate sounds the mic pres are a little harsh for my taste. A lot of folks doing field recording are gravitating toward the Fostex FR-2 modded by Oade brothers (I've used one and they're great) or the Maranntz PMD661 (Oade is working on a mod for that as well); unfortunately that would eat up most of your budget.
In the field it is very close to impossible to completely isolate the surrounding ambience from what you are attempting to capture. Plant mics can be a better option if you want to get in close, but require a lot of planning and patience. I've used plant mics to capture individual crickets and birds with mixed results, and there was minimal to fair isolation at best. I would scope out a location where the source I wanted to capture seemed to be on a regular basis and planted a mic nearby (usually my old reliable Audio Technica ATM-41 electro-cardioid condenser covered with an extra large unlubricated condom) hours in advance, ran a lot of cable (100 feet on several occasions) and then waited patiently for my subject to arrive or become active. Sometimes they did, sometimes they didn't, or they didn't make the sound I wanted to record.
Are you a one man crew or will you have help? That will be a determining factor in the gear that you choose. If you want some more focused suggestions we could use a few more details about your situation.
