Most of my herd traces back to a couple of Holstein heifers (one red, one black) that my wife gave me as a birthday present, back in '87. Dairy-type udder is still evident on a few, despite 25 years of breeding to beef bulls(Simmental, SimAngus, Angus, and now some Shorthorn) - but they raise a whopper of a calf, maintain body condition, and breed back - but few are more than 1/8 Holstein, anymore. The F1s, not so much - breed-back, on a minimal-input management program wasn't good - but with adequate forage & feed, they may work well for you.
Through the years, there have been at least a couple of 'composite' beef breeds developed, with Holstein in the mix - Hays Converter was/is a mix of
Holstein, Brown Swiss, and Hereford; RX3 was developed from red& white Holsteins bred to Herefords, with those F1s bred to red Angus. So...you're not exactly reinventing the wheel.
Recommendations to buy beef cows/heifers with the proceeds of the sale of the milk cows - either as springers or fresh milkers - are probably the most reasonable, but I can understand your desire to retain ties to the parents' herd.
All the beef breeds bring something desirable(and sometimes some less-desirable) to the table; you get to pick what direction you go - though you may want to consider what your local market 'wants' - black or smokey calves sell better here than reds or spotted/colored cattle.
Having been breeding at least some Simmental influence for nearly 30 years, I'm partial to them - but, like countrycattle, I've always liked Brown Swiss, and like what I've seen of Braunvieh and Braunvieh-cross cattle; you might consider Braunvieh in your crossbreeding program, though few of the major beef studs, like ABS, SS, Accelerated, etc., carry any Braunvieh semen.
