75,000 doesn't go far in professional development, but as a hobbyist -- doing most of the work as a solo developer, or working with a small volunteer team -- it's a reasonable budget that you could probably stretch quite far.
You could buy Unity Pro for $1,500 and then get one of the numerous RPG starter kits from their asset store for around $100 or less (I found 3 within that price bracket with a quick search: 1 ($65), 2 ($30), or 3 ($70) and there are presumably more available) to save you some development effort. Obviously you would need to look at the feature sets, check out reviews and make sure what you were purchasing was suitable for your needs, but this is a reasonably cheap way to get a head-start on your development. You would obviously then also need to learn to use program using one of the languages supported by Unity (C#, UnityScript, or Boo) and to use the editor and your chosen starter kit.
You might then look into purchasing some royalty free stock models to start putting content into your game. These are "generic" models that you don't have exclusive rights to, so they might appear in other games and may need some tweaking to be suitable for your project. Sticking with the Unity Asset Store to look for a couple of examples, a quick search turns up Fantasy Horde - Enemies as a package of 10 assorted enemies you could use in your game for $100, and Fantasy Treasure Loot Kit which claims to have "298 treasure prefabs" for $45. I listed the normal prices for these items, but both packages are currently on sale for 65% off and 50% off respectively, so if you're patient and willing to do some research you can obviously save quite a bit of money. Looking for a couple more examples outside of the Unity Asset Store, we have the Frogames CS:Warriors and Commoners package available in the GDNet Marketplace, which offers a set of parts that can be assembled to make your own characters for $150, and at Gamedevmarket.net we can find a Male Mercenary model for $10.
That's quite a lot of graphics for your game for just $305. However, if you now go back and look through all of the linked models, you'll see that although they all look good, they're not all the same graphic style and might look odd if you put them together in the same game. You would need to spend more time searching for things that match well, or spend time adjusting them (this would mean investing time learning another skill, and possibly investing in some modelling software) to make the style match. You might also need to add animations specific to your game, and are likely to have to adjust things like scale and format of stock models.
It's also likely that you might want some characters or items made specifically for your game, which would involve hiring an artist -- this usually ends up quite a bit more expensive than using stock assets, but if you find a good artist and negotiate correctly should get you exactly what you want, and potentially (depending on the specifics of your negotiated agreement) gets you exclusive use of your assets.
Note that I'm not actually suggesting any of the above as the best approach for you -- I just thought it might be valuable to provide a worked example that shows what you could potentially pull together with a reasonably small amount of money -- buying all of the above would still leave you with a lot of work to be done and still plenty more assets before you had a good, complete game, but by looking through those links you now have an idea of the quality of assets you can get for certain prices.
Before you go out and purchase any of the above be sure to research alternatives -- I just grabbed the first promising links from search results to illustrate the possibility.
Is it possible to create an RPG with Diablo-like graphics on a $75,000 budget? Yes, absolutely -- assuming you're willing and able to do the necessary work, you're aiming for something of reasonable scope, you're willing to have non-exclusive rights to at least some of your assets and you go about things in the right way.
Will it be possible for you? We can't really tell you that.
Hope that helps!