1000 USD and I am willing to buy used items if its really worth it
I would say that for people reaching grade 6, the benefit of buying a used digital (especially if you have a lowish budget) is that you are able to buy a piano with a more nuanced action: which can cope better with the requirements of grade-6 to 8 pieces.
Also, if buying a recent model used, the sampling bit-rate etc., will be much higher in an intermediately priced piano than in a lower priced model.
Now from a beginner's perspective, yes you can buy a new instrument for $1000 which will suffice.
The depreciation with digital pianos is worse than for cars.
A new $1,600 mid-level portable piano (e.g., ES920, FP90X, P515 etc) will sell used about 2 years later for a tad over $1000.
A new $850 low-priced portable piano (e.g., ES110; FP30X, P125) will sell used for about $350--$450
An intermediate level portable piano should last ----maintainance free---for about 10 years. If you do have to pay for ware-and-tear repairs due to regular intensive practice, then the cost of repair will be relatively low---typically the labour bill for 2h work dwarfs the part cost e.g., $70/h.
Ergo, buying 2-3 year old used digital piano is not a bad bargain.
As you test before you buy a used instrument, you can ensure it fully works before purchasing ---whereas new instruments get shipped by the store to your house and you get what you get.
So what I advise is to go to a store and test lots of models from low-mid-&-high priced models---get an idea of what they feel and sound like to play.
If you feel like buying an intermediate level portable or cabinet piano, you can always come back to this question and ask tips on buying used.*there are a few posts on this topic already which can be googled .