You sound like you live close to me with your cost. I would recommend 1070ti 6 card rig or a mix and match of 1070 and 170tis based on the availability of cards. You can put together a rig for 5k with no issue. The guide below I wrote will get you to 6 cards 1070 or 1070ti for right around 5k depending on the availability of the hardware which can be tricky. If you have any questions I would be happy to help.
https://theminerhub.com/mining/building-a-mining-rig-part-2-hardware
what is benefit of mixing ?https://theminerhub.com/mining/building-a-mining-rig-part-2-hardware
its not better to do with six 1070ti card on a rig ?
or its better on a 6 rig with 3 Nvidia and 3 AMD cards. or maybe on two 3 rigs with 3 NVidia cards and 3 AMD Cards.
Not much really, but yes it would be easier to just use same cards to prevent driver conflicts and more adjustments. Mixing manufacturers would actually be a major stability hit on the rig, but in a situation provided you already have both, then it can work.
its great to have all the same card yes. I would highly recommend it. Right now you cant really do that if you want to buy everything at once. The card availability forces you to mix and match if you want to get up and running quickly. I would stay away from mixing NVIDIA and AMD cards but generally speaking, if you have different kinds of NVIDIA it just a matter of updating drivers and tweaking overclock for each and you are good to go. If you can keep them all the same but its much harder to do that right now.