We have a medium budget. We also have Java programmers. We do not have a scenario!
But I don't know how to build this service.
I counted; I need 300,000 searches per month and this is difficult for us
IP blocking, Google captcha, etc. are our problems
We can scrape Google results. But we don't know how to solve the Google captcha or change the IP for a high search number per second
You need to fire those Java programmers if they weren't able to offer you a technical solution. Might as well call them VAs.
Let's use your case scenario. 100 clients who need 100 searches/day each.
One dedicated proxy can perform a google search safely every 60 seconds, tested personally times and times again.
This means that, in 24 hours, you are able to perform 1,440 searches from one dedicated proxy.
You need 10,000 searches/day (100 clients * 100 searches each).
I don't know who told you that you need to perform 10,000 in a second, maybe it was those Java programmers.
You have 24 hours in a day, you can spread them out how you wish.
This translates to less than 10 dedicated proxies.
Heck, you can even afford to rent 30 just to increase delays and be more on the safe side.
I don't know recent proxy prices, but renting 30 proxies/month shouldn't cost more than $50-$70.
Assuming you'll have 100 clients (wishful thinking right here), the least you can charge them is going to be around $10/month.
That's going to be $1,000 in revenue.
But put it another way.
You only need 7 clients(paying clients) to break even and wait for word-of-mouth to spread.
Bonus tip:
This shouldn't take more than 30 hours of development from any programmer.
If those Java programmers of yours will give you an estimate that's more than $1,000, well... I don't need to teach you how to run your business.
I'm talking about the backend part.
The front-end stuff is relative.
You can have a cheap decent design for $300, or you can spend $4,000 for some Sci-Fi UX that no one is going to care about anyway.
Bonus tip number two:
Given how the operation is going to take place, you can use already written code that resolves reCaptcha using voice recognition.
I've tried it numerous times and it works flawlessly.
This is basically your business plan.
You now owe me $200 for saving you money and time.