It really depends on the space available. If you've only got 150mm or so of height available, then maybe look for a thicker core like a 450x160x90 or something?
Forge Motorsport sell a nice one that is quite slim... 600x180x70 from memory. I bought one last year. 180GBP, which worked out to be ~$600 AUD delivered including shipping, customs and GST. www.forgemotorsport.co.uk