Please consider this carefully, reducing heart problems etc has been done only by the monitoring and breeding of health checked dogs, if you are really intent on a litter get your bitch heart tested at least and ideally hip scored and go for the best stud dog for your bitch making sure he is on the Boxer heart tested list.
In Boxers stud dogs are very reasonable prices anyway and certainly not the price of a puppy, for example we used a newly made up champion from one of the countries top kennels last year and only paid £250 and we had a beautiful line bred litter I would certainly not expect to pay any more than £300 for Multiple CC winners.
If you need a bit of help most top breeders would gladly look at a copy of her pedigree and point you in the right direction but saying that if it was my dog the bitch would have to be of a good standard before I would let anybody use him and I would want to know that the owner was quite aware of the work involved in raising a litter, on a note of caution a lady I know recently bred her 4th Boxer litter and I consider her to know what she is doing, the bitch ended up with a sudden severe mastitis and had to have half of her mamary glands taken away and she was left with a large litter to hand rear and the bitch was on a drip at the vets where it was touch and go for a while. She does have a lovely litter and luckily the bitch survived but also she has an horrendous vets bill. If I were you I would think very carefully!