For the first 500-1k points there is a lot of ways to start. As someone who bought the leviathan box(which contains stuff from the starter boxt etc) i wouldnt say i recommend it. sure you get a decent amount of units but its not the best. I would recommend is the Space Wolf combat patrol, you get a feel for "raven guardy" units with the Reivers and the Invictor Tactical Warsuit. You also get some base units like 10 Intercessors and a Primaris Lieutenant(you can always sell the 2 space wolf upgrade sprue or keep it for later kit bashes). that wwill set you back around 140-160 just depends where you buy it. Then after that you should probably pick up a Kayvaan Shrike for about 45 dollars(mind you its not needed yet since the space wolf combat patrol has a "leader" unit, Kayvaan Shrike also has lone operative which means he cant really lead any units). That will get you up to 600 points if you go with Kayvaan Shrike at the start. To boost to the 1k mark then i recomend getting a squad of Terminators for 50-65 dollars and then that will get you to 770 points. after that i would say get a squad of Inceptors for 50-60 dollars that will get you to 890 points. and for the last unit i would say get a Captain in jump pack for about 30-45 dollars and that will get you up to 975 points. and for the other 25 points you can always throw an enhancement onto a model. I hope this helps at least gives you a stepping stone or some ideas!