Always drainage. Even semi-aquatic plants - in nature there is water movement through the root systems which introduces oxygen. There's no such thing as too much drainage, just too much drainage for a reasonable watering schedule.
Container material isn't too big of a deal. Clay's advantages are weight (no toppling over in wind), porosity, and sturdiness (roots aren't disturbed when the pot is moved). Take care when moving your plastic pot so it doesn't jostle the roots too much, and don't let the soil dry out (for the obvious "plant needs water" reason but also so it doesn't fall over). Best is fabric, but that's introducing a much greater deal of attention or materials.
I wanted to ask about where you said "topsoil". If you filled your container with actual topsoil, this will be a problem. Topsoil is an amendment for ground or bed gardening, it's not a potting medium. Even when put in the ground, it has to be mixed with either native soil or a host of other ingredients to promote drainage.
My recommendation: Take it out of the pot, drill drainage holes, gently get as much soil off the roots as you can, gently wash off the roots, repot into a potting mix. (Standard Miracle-Gro Potting Mix is fine if you don't want the hassle of making your own.)
Personal note: I love lilacs. I have a 8'x8' patch in one corner of my lot, and six or seven 15'+ tall ones scattered on the wood border. When they bloom my backyard smells ridiculously amazing. AND when they have to come down (had to take down a couple twenty-footers two years ago) they provide incredible wood for small projects - twice as hard as oak and the heartwood is an amazing pink-peach color that contrasts the bone-white sapwood. So - good luck with that plant!