Don't buy a car that you could easily win. Buy a car that will allow you to win a bunch of useful cars that are eligible for events that the car you bought isn't eligble for.
For instance, since you could use any car to win a MX-5 in the Clubman cup there is no reason to buy one, better to win it with something else.
Here are the beginner races and the cars you win:
Sunday Cup - Toyota Sprinter Trueno GT-Apex
Clubman Cup - Mazda (MX-5) Eunos Roadster
FF Challenge - Toyota Vitz RS 1.5
FR Challenge - Nissan Silvia K's S13 1800cc
MR Challenge - Toyota MR-S S Edition
4WD Challenge - Alto Works Suzuki Sports Limited
Lightweight K Cup - Mini Cooper 1.3i
Stars and Stripes Grand Championship - Chevrolet Camaro SS
Spider/Roadster Cup - Mazda Roadster RS
80's Sports Car Cup - Mazda Savanna RX7 Infini III
NA Race of NA Sports- Honda CRX Del Sol SiR
Turbo Race of Turbo Sports - Daihatsu Mira TR-XX Avanzato R
Tourist Trophy Audi TT Race- Audi TT 1.8T Quatro
Legend of Silver Arrow - Mercedes SLK 230 Kompressor
Altezza Race - Toyota Celica SS-II
Vitz Race - Toyota Vitz RS 1.5
Type R Meeting - Honda Civic SiR-II, Honda Civic Type R
Evolution Meeting - Mitsubishi Lancer Evolution IV GSR
Beetle Cup - Volkswagen New Beetle RSi
Grand Turismo World Championship - Toyota Celica GT-Four,
- Mazda Eunos Roadster,
- Mistubishi Lancer Evolution VI GSR,
- Nissan Skyline GT-R V-spec II
I recommend a Lancer, specifically: LANCER Evolution VI RS. Cheap, powerful, adjustable rear wing, easy to drive and can fit rally tires.
(Buy) Lancer
(Win)
Sunday : Sprinter Trueno -> 80's cup
Clubman : MX-5 -> Roadster, FR, NA
4WD : Suzuki Alto -> LightWeight
Turbo : TR-XX -> Sell
Evolution : Lancer -> Sell
Grand Turismo World Championship : GT-R -> Use later
Then buy an Integra, specifically: INTEGRA Type R 98 spec.
(Buy) Integra Type-R
(Use to Win)
Type-R racing : Civic -> Sell
FF series : Vitz -> Vitz
But neither of these cars, although cheap, can be purchased with your initial credits so they would be my second and third car choices. My first car would be something eligible for races that the Lancer/Integra combo doesn't already cover. So either the "PT Cruiser" or the VW because the PT can enter the US championships and the VW can enter the Beetle racing.
This leaves the following Beginner Races, subtract one depending on which way you go in the PT/VW choice:
MR Challenge
Stars and Stripes Grand Championship
Tourist Trophy Audi TT Race
Legend of Silver Arrow
Altezza Race
Beetle Cup
So with only 3 cars purchased you can enter 15 of the 20 races.
Win everything you can and leave the five remaining races for now.
The last two races of the rally level are on wet street circuits and any car is eligible. Use whatever money you have to mod your best car to the level it will take to win the SS Route 5 Wet II series.
SS Route 5 Wet II - Subaru Impreza Rally Car Prototype
The Impreza comes with rally tires fitted and is eligible for ALL the other rally races. Win them all, finish the entire rally level and win the Escudo.
You will have tons of money and a garage full of race-prepared rally cars, each of which can be sold for more than 75,000cr. The Escudo sells for 250,000cr but you can't get another one so best not to sell it.
Efficiency will cease to be a serious concern.