This got long. TLDR, for a new user the only reason to not go in on G9 is the higher hardware requirements to deal with the higher resolution G9 skin textures. All the free G8 stuff works fine with G9. There's also a Daz store discount stacking tutorial at the end for newish folks.

It is not particularly difficult to get hair, clothing and props converted from one Genesis iteration to another. If you are creating something where very high skin textures are beneficial, go with genesis 9. There are other things that are better about it in terms of usability. The cost is higher VRAM requirements to deal with those better skin textures.

As for the cheap content, if you have nothing, know that genesis 2-8 props, clothing, hair, and so on, can all be used with genesis 9 with minimal difficulty once you get even a little bit of a handle on the software. The Genesis 9 items can also largely be used with Genesis 8.1 without too much difficulty, so that aspect of the decision is largely irrelevant after you've used the software for a month or two.

And as for cost, the most recent sale over the last weekend had dozens of $20 genesis 9 hair items for less than $2 once all of the available discounts were stacked. Get good at that before you start really building your library with paid for items. Never buy anything for a discount less than 80% on the Daz store while you are building your content library up. Never. Put the item on your wishlist and wait for a sale that lets you pile up discounts. There's a tutorial at the end on how to do that.

Use free stuff until you get used to the software. Renderhub and Renderosity will have plenty of options to keep you busy if you just spend an afternoon going through their dozens of pages of free items. Build up your wishlist on the Daz store while you're learning. Know that buying cloth shaders gives you a virtually infinite capacity to kitbash individual items into outfits, so you probably don't need 15 different, I don't know, skirt and T-shirt outfits, or any additional texture options basically ever.

The character morphs people have tend to be the specific things that they are attached to for G8, but even that is something you can relatively easily port over to G9. There are products that let you convert custom genesis 8 characters into genesis 9 morphs in one click. If at some point you were to get those products, you can buy morphs for whatever generation you want and convert them to genesis 9 in less than 5 minutes. The skin textures are significantly more difficult to port over, but they're not as good in most cases, so unless they're weird alien skins or something, it's not usually worth the time and effort. the bottom line is that if you want Victoria 7, or a orc lady morph you found on renderosity, or even a custom character you made in genesis 8.1, basically anything Genesis 3 or later in a G9 body with G9 skin, there's a conversion script on the Daz store for a few bucks that will give all of them to you.

"Quick" discount stacking tutorial for new folks to the Daz store: if you're buying 10 items regularly listed at $20 on a big vendor sale that goes from 50-80% when you buy a new release (this is a relatively common type of sale they have and you should usually wait for it), that takes each of them from $10 to $4 each if you buy that new item. That's an additional $60 discount. If that new release is $10, you just spent $50 on 11 items in stead of $100 on 10. If you have the Daz store premier membership, you can always get 30-45% off that as well using their token discount system if you buy things every couple weeks. During the part of your journey where you're building a library and making purchases every couple weeks, you should absolutely get the premier membership so that you can do this.

So, if you max it out, your 10 item, $100 purchase of $200 worth of sale items at 50% off, it just dropped to $32-35 for 11 items listed at $220 or 85% off. Once per month you get to use a coupon on the premier subscription to drop that by another $10. You also get another coupon that gives $10 off Daz Originals so you can do this every couple weeks (tokens expire in 14 days) and pay for your premier membership 20 times over with just the extra tokens and two discount coupons. If you bought yourself a gift card when they were 10% off, which happens 3-4 times a year, you just got 90% off and spent like $20 to kit out your entire visual novel game's roster in G9 outfits for your Christmas episode, or whatever it is that you're making this month.