Lots of people choose a tup lamb in the fat ring, use him and sent him off fat. Practically zero cost, and usually does a perfectly good job as a terminal sire.
We usually buy pedigree or specifically bred-for-breeding tups, as we breed our own replacement ewes and aim to have 80% of our fat lambs be in the top quartile of price/conformation.
We bought an aged pedigree Charollais tup for £450; he'd cost £2,200 as a shearling. His owner, a prizewinning breeder of pedigree Charollais tups himself, was selling this tup now as he'd now used him as many times as he could, and now had too many of his daughters in the flock to be able to use him again. As his genes got into our breeding ewes, our average conformation scores lifted a whole point, which makes 5p/kilo deadweight difference. 300 lambs x 20kg x 5p = £300 per annum increased revenue - forever (provided we don't do something silly, of course.)
So it depends on what you're wanting to achieve. Because you're creating your foundation flock, I'd be inclined to get the best tup you can. But buying an aged tup is usually a good way to get great genes for a fraction of the cost of a tup lamb or shearling.
Once you have your breeding flock established, you could do the buy a fat tup lamb thing, and only pay more money for a more predictable tup when you need some replacement ewes.