If youre just going for looks springs alone are fine, if youre concerned about handling as well then upgrade springs and struts.
Don't just take my word for it, check these links by people who know allot more about suspension systems than I do.http://www.koni.com/_cars/frame.html
"The use of lowering springs requires special damping characteristics: the mere application of stiffer springs together with ordinary shock absorbers often has a negative impact on road-holding and comfort, and may even lead to dangerous situations."
And this from SportCompactCar's site about their 2K Project Celica: http://www.sportcompactcarweb.com/projectcars/0106scc_projcelica/
"We accomplished our goal of good looks with a set of Progress springs, but without stiffer shocks, we could do nothing about the lack of damping. Riding lowered on the stock shocks, the car felt slightly disconnected--like it was hovering a few inches above the ground. All too frequently, however, our hovercar would seem to scrape the ground as it crashed into the bumpstops. Despite rear springs that were proportionally stiffer than the fronts, the car seemed to lose its playful handling with the lower springs as well. This is, unfortunately, pretty normal for a car with lowering springs.
and finally this:https://www.celicahobby.com/ubbthreads/thread.f_253784_0_collapsed_5__1.html
Sure, coil-overs would be best, but they will cost you between $1200. and $1400.
Unless your an experienced mechanic with access to lots of tools (especially a spring compressor) I don't think you want to try putting springs in yourself.