Thankfully beer kits have come a long way in the past decade, and the "twang" is less common nowadays.
In terms of eliminating off flavours:
1. Pay a little extra for a good quality kit.
2. Avoid pouring boiling water onto the liquid malt extract, I stop the kettle half way before its boiled.
3. Remove chlorine from water using half a campden tablet, I do this a couple of hours before adding yeast.
4. Resist the urge to keep checking it, keep the lid on to avoid introducing oxygen.
5. Ignore the instructions and leave the yeast for a full 2 weeks to ferment and tidy itself up.
6. Keep the bottles cool and out of sunlight.
in terms of quality kits, the following should all make a good brew:
1. As mentioned previously, the Youngs AIPA is great and comes with 100g of hops.
2. Love Brewings "Presidents Sierra" makes my personal favourite.
3. Mangrove Jacks "Juicy IPA"
4. Mangrove Jacks "Pink Grapefruit IPA"
Finally, if you prefer your IPAs hoppy then consider buying some extra dry hops. Some kits only contain 50g, which is a touch low for my taste buds.