The words Simple, Lowcost, and Portable are not usually used in connection with 145Mhz repeaters.
First of all we will disregard the licencing issues, I dont know how it works in your country.
Technically the first consideration is the antenna(s) and the Tx/Rx frequency spacing.
The usual frequency spacing is 600khz, and if you want to use a single antenna for Tx and Rx you will need a duplexer with about 80/90dB isolation between transmit and receive ports.
Such duplexers are not simple, lowcost or portable.
You will need separate transmitter and receiver.
The receiver will need good overall screening and good filtering in the RF stages.
The transmitter will need good overall screening and low phase noise to prevent swamping the receiver.
Depending on the application, you will need some control logic and a callsign generator, there are various designs around on the web.
Jim GM3ZMA