Sheesh. I use theatrical fresnels weekly for photo and video shoots. If the wiring is good, they're fine. I have L&E (altman) theatrical fresnels I bought 16 years ago. I've converted some to HMI (well, HID). They're great lights.
I have some 3" chinese Arri knockoffs as well, 300 watts and very useful. I have some 6" that can be lamped from 300 to 1k. I have 1.5" inch units that take 100 - 150 watts. All very handy, all have their uses.
Go to Harbor Freight and get a couple Router Speed Controls, which are 1k dimmers (good for up to 1000 watts). Household dimmers are generally good to 600 but will burn out fairly quickly at that level. (Dimming for color will make the light much warmer though).
The thing about fresnels isn't just aiming light at something - the lens provides a "just right" mix of hard and soft. Nothing else is really like it. It's nothing at all like diffused light. Using a slide projector? That's some very hard light, and have fun sticking it on a stand to aim it. (If you want hard light, get a used Leko or ellipsoidal light. They're all over the place and affordable if you hunt around). Generally you'll use a big diffused source (softbox, panel setups, bounce) for overall soft light, and fresnels as kickers or rim lights or to add some sparkle - that's certainly not a hard & fast rule but seems to be the way lots of folks use fresnels.
And the comments about burning plants and stuff? Do you need lights an inch away from the plants? Are you looking for a 2000 watt 16" fresnel? No? Fresnels are usually at at least some distance to get the effect of the lens. You'll be fine.
LEDs? Fine if you want to diffuse them for soft lights. There are no LED fresnels as of now that you want to pay for, unless you have a few grand to burn.
Poke around on eBay - there are usually many 6" and 10" altman/L&E units… but some older units use twist-in projector bulbs; newer stuff uses bipin globes that are a little cheaper and in my experience, more robust. The chinese fresnels are really not bad, but I'd check the wiring and grounding on 'em - really inexpensive, brand new, and there are now US distributors.
Also google Pyramid Films, they often have a lot of stuff, or call them if nothing's listed. Google Cool Lights, they manufacture in China with US sales and service. More pricey that straight-from-China knockoffs but a great company - I have several of their tungsten fresnels. If you buy a used theatrical light, you may have to ditch the c-clamp and get a spud - search for a TVMP adapter. And get the Manfrotto, not the Matthews - the Matthews needs tools to get on and off the stand, the manfrotto has a big t-handle. You'll also need to replace the theatrical distro plug with an edison plug from the hardware store in most cases.
The best units are mole-richardson (old school, cast and stamped metal) and Arri (modern, extruded aluminum, very pretty fixtures) - those are hollywood/filmmaker go-to brands. L&E (Altman), Lee, Berkey-Colortran, Desisti, Century - all kinds of stuff out there used. Older stuff may be heavy cast metal, or stamped steel cans. There are some gorgeous space-age retro units out there if you're into that. Regardless of brand, as far as the light looks on your set? I can never tell a difference. It's more about the reflector and the lens.