Read this first...
The OEM plenum is the weak link, if you need to do things incrementally I'd get an after market plenum first, then a TB.
A 75mm TB is a bit of overkill on an n/a setup, however in practical terms it will only affect things at WOT and lower RPMs where there may be a bit of bogging--backing off a bit from WOT will get things moving again in this instance.
I got a 70mm and figured that if I was to go f/i then buying a 75mm TB for $90, and selling the 70mm TB for $50 on eBay, would be a $40 net cost--a very small chunk of what it will cost for a f/i setup...
As to needing a tuner, there won't be much to show for a TB/plenum without a tune--a bit better response up to redline is about it. The OEM 87 octane tune is very conservative, I recommend doing an exhaust upgrade first, then a tuner, then gears or TB/plenum, if you want butt-dyno detectable increases...