@Drayk: I’d advise against purchasing Loki, for several reasons: his parts are easily farmed from the Neptune boss, the Hyena Pack; if you don’t have access to that node you can ask in Clan chat and someone can “taxi” you there and help you with the fight. Likewise, the resources needed to craft those parts aren’t hard to collect and, again, you can ask for help for the rarer ones (Neural Sensor, Orokin Cell).
Another reason against just purchasing Loki is that to be effective at what he does, Loki is very dependant on having a good set of mods: in Warframe, all the gear (be it weapons, companions, or the warframes themselves) are kinda like an empty vessel; you need to equip and rank the appropriate mods to realize the true potential of each item.
On the other hand, though there are very few things that are Platinum-exclusive and can’t be obtained just by playing the game (trading for Platinum with other players nonwithstanding), at the start it can certainly help to purchase another warframe that you think you may like, to have more options apart from your starting gear. In this case, there are a few recommended ones, either because they are too grindy to obtain by normal means (Mesa, Equinox), there’s too much RNG involved (Vauban, maaaaybe Ash / Oberon), or they are locked behind a quest that won’t be available to you for some time (Mirage, Limbo, Chroma, Atlas).
But if you are really interested in Loki, then I’d suggest you buy the Loki Prime parts and blueprint from another player***** and craft him instead, it’ll probably cost you less than purchasing Loki from the Market and the Prime variant is a strict upgrade over the vanilla one (it has higher energy capacity and 2 additional polarities). You’ll be missing the pre-installed Orokin Reactor and warframe slot that purchases from the Market get, so that’ll be some additional 40 Platinum on top of what you pay for Loki Prime, in case you want to compare prices.
[SIZE=2]***** To be able to trade with another player, your account needs to be at least Mastery Rank 2. Also, you can’t trade away the starting free 50 Platinum, so keep that in mind.[/SIZE]