The Su-27 may be a bit too ambitious for a first aircraft model, considering the large number of parts (i have no idea of what "Gunpla" is or how it relates to modeling). Again not 5th generation (but then, is the Su-27?), but you may want to look into recent Academy 1/72 kits like the F-16 and F-18 (and then the F-4). These kits are very accurate and detailed, and the fit is excellent, having many "press-fit" parts,
They would be fine mules to become familiar with what may be new materials and techniques, and you'd have a great looking model ... as well as a frustration free experience.