If you have the time and resources you will do better rearing the lambs to slaughter weight and getting them butchered yourselves, selling boxed lamb. Generally Shetland store and fat lambs are undervalued in an auction ring, IMO.
Because these sheep are best slow grown and slaughtered as hogget, your numbers will grow surprisingly fast after the first 18 months.
Your 5 ewes should have 7-8 lambs. They'll still be with you when the original 5 lamb again in 2017, so that summer you'll have 20 sheep. Perhaps 3 or 4 of the first lot of lambs will go that summer, the other 3 or 4 be kept on as breeding sheep. So after lambing 2018 you'll have 7-8 ewes and their 12-13 lambs, plus the 7-8 hoggets growing on from the previous year - 26-27 sheep in all.
Yes, you'll need more than that for 20 acres, but because the numbers rise dramatically once your first lot of ewe lambs join the flock, do do the sums to work out how many ewes you need to start with for your eventual desired stocking level.
Do either of you spin?

If so, you'll love using your own fleece - and, more importantly, you'll know what to look for in a fleece and make a good job of selecting those to sell / get spun into yarn

.
Shetlands are capable of carrying lambs to a larger tup, yes, and thereby producing an acceptable and market-friendly lamb. What they do on Shetland is cross them to a Cheviot, then keep the ewe lambs on and put them to a 'terminal sire' such as a Suffolk or Texel. The Shetland x Cheviot makes a good ewe; we've actually considered bringing a batch down from Shetland for our own commercial flock.
Whether you'd want to do the whole three-tier breeding on your 20 acres I'm not sure. It means using three tups - a Shetland, a Cheviot and a terminal.
You can also put the Shetland directly to the Texel. I haven't done this myself, but I believe it will, over several lambings, cause the ewes' bellies to sag somewhat, so others I know who do this put them to a Texel or other larger tup for perhaps three crops, then back to the Shetland for replacement Shetland ewes until they're past breeding.
If I were going to try this, I think I'd be more inclined to use a Charollais than a Texel - smaller, lighter lambs at birth, but grow and fatten well. You might get more crops this way.
Another interesting cross is to the BFL (Blue-faced Leicester) for a 'Shetland Mule'. I have a few of these now; they are good sheep, bigger and stronger than their Shetland mums, and with fleeces which can be very appealing to spinners. (I say 'can be', because good ones are fought over! but they're not
all that good.)
What made you choose Shetlands for your first 5? Do you fancy showing them?
If you like the idea of showing, and of breeding pure bred stock, might you consider a different breed? There are a lot of Shetlands about now, they're easy to buy. Whereas some of the breeds which are still rare (a) need help and (b) are harder to find to buy so perhaps fetch better prices.
There are plenty of larger sheep on the
RBST Watchlist too, so you could pick a breed you like, have larger lambs to sell, and perhaps also cross with the Shetlands for replacement ewes.