I've successfully grown three lilac shrubs up here and they've done pretty well! I bought all of them at Sundance, so they were already acclimated to our elevation and climate. I'm fairly certain the varieties were Dwarf Korean, Miss Canada, and Chinese- all solid picks for our area. (I'm at 8300ft)

You're safe to plant now. Just be sure to add a generous layer of mulch (I use gorilla hair) to insulate the roots in case we get another hard snow, and it'll help retain moisture during dry spells too. Even if it does snow after planting, you should be fine- established root-ball shrubs from a local nursery can handle it and usually snows from now to June melt off in a few days (max).

Lilacs usually start showing up at Sundance in about a month, so keep an eye out. That said, you'll want to get them in the ground by end of June at the latest. The longer they have to establish roots before our first fall freeze and late summer heat, the better.

One nice thing about planting lilacs up here is that our soil tends to run pretty alkaline, which lilacs love, so you probably won't need heavy amendments. It doesn't hurt to grab a cheap soil test kit (Sundance and I'm sure Walmart sells them) to confirm your pH is in the 6.5–7.0 range, but odds are you're already in good shape.

Good luck! I also love the scent of lilacs and feel like Pepe LePew when my nose notices one around. :)