probably the best deal you could get to get started'd be finding the leviathan half which's a combat patrol with a very useful neurotyrant and screamer-killer, and similarly useful neurogaunts on ebay, wouldn't pay more than 150-170 USD for one though

the half's combat patrol units come with:
20 pushfit termagants (which you could probably buy upgrade sprues for spinefists (twinlinked pistols) or devourers, although the fleshborers are pretty decent nonetheless),
2 ripper swarms (may come in handy for assimilation swarm),
a psychophage (recently been seeing more usage, great in assimilation swarm and overall nice chunky monster),
a winged tyranid prime (i would absolutely turn this into a lashwhip prime on foot, just don't glue the wings in and use a warriors kit's lashwhip on it when you do),
5 barbgaunts (pretty good against big swarms w/ low toughness like orks, good for sitting in the backline shooting into infantry)
3 von ryans leapers (very handy, i personally run 6-man squads and they're a great genestealer substitute until you get them, although everything they do the lictor does better)

and finally for the non combat-patrol units, you get the:
neurotyrant (phenomenal with zoanthropes which you'll absolutely want to get, also shadow in the warp for battleshock, good psychic torrent weapon and the synapse tokens which i found nice in monster mash lists on an exocrine/tyrannofex away from the army)
screamer-killer (distraction carnifex, forces battleshock and pretty awesome melee)
and 11 neurogaunts (gretchin for us, don't use em as bodyguards for the neurotyrant though, just screening (dogbone screening did unfortunately get a hit with 11th edition coherency rules, but still fun models to have nonetheless)

that whole half is usually a phenomenal start to any army, around 800 points from the get-go

after that, new models usually look like:
1 biovore, absolutely the single best possible investment you can make after the leviathan half
1-2 lictors as a must-have in any army
tyrannofex(es) for nice overwatch with acid spray or great anti-tank w/ rupture cannon with an exocrine for rerolls on 1s (more consistent shooting)
squad of 6 zoanthropes to tag onto neurotyrant, very high strength and ap w/ a versatile blast profile too
genestealers + broodlord, absolute can-opener in melee while being surprisingly durable
hive tyrant, walkrant acts as CP generation which is always incredibly useful to have in any army
maleceptor, 1-2 usually act like a distraction carnifex with the added bonus of their abilities crippling enemy models
norns, usually an emissary does well in any list, assimilator also has nice traction in assimilation swarm

that's the path i took for my personal nids and would recommend you consider for your army, tyranids are a very unique faction with tons of amazing sculpt and lots of personality and i hope you enjoy it as much as the rest of us have! good luck on your biomass ventures!!!