How much ride clothing do I need for IM training?

If you're inside, on the trainer, ride nekkid

If you have zero cycling specific clothes, start out buying two different pairs of bike shorts.  Over the next 6 months, you'll know what you do/don't like about either pair and can buy a third pair that is more likely to feel right to you.  You can ride in t-shirts (they're not as aerodynamic as a cycling jersey and they lack the pockets, but they'll work). 

I plan on buying a pair of cycling shorts and a pair of running shorts every year.  I figure they last 2-3 years, so I have 2-3 pairs to rotate and I'm replacing the oldest or rattiest pairs with the new ones. 

Some folks do all their bike training in tri shorts others like bike shorts.

I'd suggest getting one pair of each; a bike jersey or two (the three rear pockets are great when doing long rides); running shorts 1-2 pairs. Some like running in running shorts others like tri shorts or compression type shorts.

One thing I find is the price you pay for gear typically reflects quality especially with tri or bike shorts.

I really like Oomph tri shorts with distance pad. They have 3 different types with that pad. For me they are great for IM training and racing. They tend to be a little more but last well and may be cheaper in long run.

There are often great deals on line if you shop around.