I have a bass tracker pro team 175 TXW with a 25 hp 4 stroke mercury for small private lakes with a horsepower limit. However, for the larger lakes I prefer to fish more I want a larger motor so I can get above 20 mph top speed. I want to get a 75 hp 4 stroke mercury. Should I buy a used motor or get a new one, hoe much would a used one vs a new one cost, and how much would a boat dealer charge to install the new motor into my boat? 

Thanks and have a good day 

The only way you could get an accurate estimate of the cost would be to contact the boat dealer who might do the work for you. The cost my dealer charged me may not be close to what you might be charged.

I had a new 150 mercury priced for me in Georgia, Install and motor and they said it would be around 12,500 so I'm guessing around 7500 to 9000 for a new 75.  Just a guess though

Probably cheaper to sell what you have and buy a boat with a bigger motor. 

If you are going to buy motor only, consider Suzuki.  My 50HP Suzuki was like new after 18 years, always started instantly and cleanly even the first start of the new year,  and was quieter than my new 60HP Merc, both 4 strokes.   Since Suzuki has no connection with a major boat company, you generally cannot get them as a package.  But they are great engines.

My folks got a new 150 4-stroke Suzuki this past spring and they paid $15 grand for it, plus professional installation.  They did get $3500 credit on a trade in for their old 140 4-stroke Suzuki though so its worth asking if they'll take the old one on a trade in.