Personally I think it's worth it to buy 290HQ from the market board, but 850-870 sounds like it would be a set of 290NQ so it should be fine for gathering red scrip at least. Getting to 600GP is essential as getting one more item out of each node significantly raises your farming efficiency.
I bought a set of 290HQ off the market board when I dinged 70 wearing level 60 Ironworks stuff. I remember I could farm red scrips fine but yellow scrips was very inefficient. But I pushed on with my red+yellow farming route since I needed the red scrips for the folklore books and I needed the Custom Delivery rewards for my crafting classes. By the time I got all my yellow scrip gear for BTN+MIN+FSH, my 290HQ set was ready to convert to materia and they basically paid for themselves. Now I wear overmelded 290HQ that I made myself. So I think going from 290NQ to 300 yellow scrip could be doable. You still want to meld at least the existing slots at least with Grade 4 materia (100% recover success) though.
The key factor I found that made yellow scrip farming feel adequately efficient was getting at least 80% raw gathering rate and 700GP. I would spend 600GP on actions and 100GP to raise the gathering rate to 95%.
Also, if you're not leveling crafting at all, you should consider just buying the offhand tools, the belt, and all the right-hand side accessories in HQ from the market board, because they are currently the best pieces you can get anyway. They don't go higher. So unless you're waiting to craft them yourself, otherwise you will have to buy them off the market board sooner or later. Then meld the accessories with at least +6GP (on either the earring or the neck piece you'll need to go to +8 GP, but don't do that on NQ accessories) and you should be hitting 700GP. You won't get 700GP if you're using NQ accessories and I don't know if the NQ armor have enough GP room to meld, but it does mean you won't be melding Gathering and Perception, making your stats sit on the minimal line. To be honest, I never planned around the NQ stats so I can't really offer any good advice there.
PS: just a reminder that overmelded 290HQ will have higher stats than the 300 yellow scrip gear. That's why the 300 gear is rarely discussed beyond the mainhand tool and body pieces which can be used in an easier 290HQ build that can hit 1200 Perception after eating food. But if you want to be farming folklore nodes and getting HQ out of them without eating food, then you'll end up full 290HQ with lots of quad-melding to hit that 1200 Perception threshold. But you should't really worry about this as 1200 Perception is only required for getting HQ items off the folklore nodes. You can get NQ off them anyways without 1200 Perception. For now, unless you're ending up getting only 1-2 collectible items per node, your gear is probably going to last you fine until you get your yellow scrip gear.